This topic is intended as a hypothetical What-If to throw players a curve-ball in terms of lore/encounters and to give GMs another tool, it obviously does not line up with the presentation in the PB RPGs (and other media), but can also be ignored as is but used as inspiration for "new" Zentreadi hardware.
Main Topic:
Could the Ground Mobile Unit/MTA-Titan in use by the EF (REF in 1E, UEEF in 2E) actually be a converted Zentreadi "armored vehicle" that was converted for use by humans, similar to how the Z-3 is a human retrofitted version of the Zentreadi Cyclops in the 1E RPG (where they don't make full use of the available volume by the description, but that's a topic for another day)?
I do believe the main question to determine if this is even possible would be to ask if you could fit a Zentreadi into any of the 3x crew/troop locations used by the UEEF: the "bridge" section, the "Mecha Bay" section, and the wheels.
Now both PB Editions have different dimensions for the MTA-Titan, and assume different "average" heights for the Zentreadi. So I will look at both editions separately when answering this question, but I will be using the same lineart to take measurements from (image is various profile views of the MTA-Titan found in the 1E REF Field Guide pg41). The "bridge" and "mecha bay" appear to be of similar size, though the "mecha bay" appears slightly shorter, though I will also admit I am treating them as basic cubes and not the irregular shape they are, so there is some "negative" space being calculated.
After taking the measurements of the basic dimensions for the MTA-Titan in its "box" mode, and measuring the sections in question to get a ratio between section and overall, I calculated the resulting size of each section for each edition of the RPG and calculated the resulting volume and compared that to a giant Zentreadi for that edition assuming it's giant size has similar ratios to human (ie average shoulder width would be 1/4 the height, and length would be 2/3 of the width).
And the Results are based on Volume:
-Bridge & Mecha Bay: 1E (Yes) and 2E (Yes)
-Wheels: 1E (Yes) and 2E (Yes)
There is actually room for multiple giant individuals at the respective "average" heights for either edition (1E is 17.37m, 2E is 11m at tall end for Elite-sub-type), though again this includes negative space that should be ignored and you need additional space to operate controls. In an attempt to address the negative space issue I went back and...
Now there is an image in the 1E Sentinels RPG (pg115) that has a side view cutaway of the mecha-bay and bridge module. The Mecha-bay includes a silhouette of a human for scale. The profile view can be broken down into 3 basic shapes (2 rectangles and a triangle) ignoring the rear bulkhead wall (thin rectangular) and top "cabin" (irregular shape). Assuming an average height of 1.8meters for the human silhouette reference, and taking various measurements for each section and totaling up the area and then multiplying that by the resulting width of the section by earlier calculations to get a new volume:
-1st Edition: Yes
-2nd Edition: Yes
So even accounting for the odd shape of the bridge and mecha-bay sections (which seem to suggest a giant's seat), it would seem that yes a (giant, presumably armored) Zentreadi can fit into any of the human crewed sections based on volume prior to any assumed conversion process. And do so quite easily in terms of volume, though in actual practice you are likely only looking at 1x per section (including the wheels), though in 2E you might be able to push that to x2 per section (maybe even more in the wheels) owning to the editions smaller average size used.
This means that if the MTA-Titan/GMU was a Zentreadi armored vehicle you could use the basic dimensions as is, along with the other game mechanical values, and just make necessary adjustments for its Zentreadi operators (altering size of hatches and purpose of modules). The weapon systems could remain unchanged, or they could have been retrofitted with "newer" human designs. The main sticking point IMHO at this point is how would the Zentreadi design have used the wheel cavities? Where they used to transport an infantry solider around in their own isolated bay? Was it a mine laying dispenser? Was it some other type of weapon station? Cargo? Another possibility is the existence of mission specific variant configurations.
Another issue is transporting it to the battle zone, I would estimate you could fit 1x MTA-Titan into a Re-Entry Pod (in 2E, 1E you might be able to get 4x, again this is due to the various size differences for some things between editions).
If we made the real world vehicles (Abrams MBT, hull only for length) and Bradley IFV) sized to be piloted by a giant Zentreadi: the 1E RT RPG Zentreadi equivalents are pretty close to the size of the GMU/MTA-Titan (talking only +/- a few meters in any direction), the 2E RT RPG Zentreadi versions would actually be about 1/2 the size. So in theory the size of the GMU would fit for a "conventional" platform in the 1E RPG, not so much for the 2E RPG where it would be oversized IF everything scales proportionally (which I doubt it will). The GMU/MTA-Titan's size IS smaller than some mobile excavators in the real world.
See Spoiler for resulting dimensions of the relevant sections based on measurements, ALL of which should be considered approximate and not the precise actual value
And given I had the measurements available I thought I'd check to see if the Mecha complements in each edition would fit into the Mecha-Bay as listed in each edition using the various measurements available, so if you're curious see the second spoiler.