Not so big one, please. My credit card really has its limit. And Bill Gates is not my father.
Actually, that's already the case, however as with most things there is a caveat. You see in the campaign script, some missions have special 'switches' if you will to enable/disable specific things.
For instance, when playing PE, your Panzer Grenadiers won't be able to access their build menus all the time depending on the mission, or sometimes specific items may be omitted on a given level. The same holds true for the British, the HQ command truck has the Allied Engineer Infantry unit enabled in the spawner list under slot 7, and the unit itself is plenty ready to go, but on level 1 of the British Campaign for instance anything but the scripted options are disabled.
This isn't to say it couldn't be modified as well, but that's actually going to alter the campaign itself and I really prefer not to monkey with what already works (I can screw things up perfectly well on my own without borrowing Relic's stuff )
Somewhere there is a script disabling specific things and that's the cause of that, but even swapping around which unit-calls to make, the game uniformly shrugs off engineer infantry for one reason or another.