I completely agree that the main book should be an RPG first and a TTG never.
But a good RPG based Vehicle combat system in one of the expansions like the Mechwarrior companion, or some type of RPG vehicle book would be nice for those of us that don't want to pull out the TTG and waste our entire gaming session on one Mech combat would be nice.
Given the nature of BT, a decent RPG based vehicular combat system should be in the core RPG books.
But that doesn't change the issue that Mech scale combat should be seen as largely out of bounds for an RPG. There should indeed be a system to let you use your character within a BT TTG, but if you are going to do that then the game should simply tell you to use the boardgame.
What does the RPG character need?
Gunnery skills and piloting skills likely need a conversion mechanic so the character can use the TTG modifiers.
Initiative becomes the appropriate Tactics skill. Thats RPG based.
Rolls to avoid shutdowns and ammo explosions? Technician or Computer skills. Again, RPG based but a conversion system to translate the Target Numbers to the RPG.
Other skills? Comms and Sensors. Again, RPG based.
Morale check? Leadership
And so on
That's not a lot of room needed, at least for a basic system. Most skills van be used as RPG skills, even on a TTG setting
Going the other way? Mech to person? The person dies. You get hit with a tank shell, you don't need to convert damage.
The RPG should be just that....an RPG. And trying to shoehorn battlefield and tactical combat and Mechs and all that has never worked.Trying to do so is a big reason why BT RPGs don't work that well. If you want to play an RPG, you aren't likely to want to play a TTG. Even something as basic as a Trial of Grievance can sideline the entire rest of the party for an entire session or two.
A BT RPG needs a decent RPG based vehicular combat system. But it should also stay away from Mechs and Mech scale systems. That should be left to the TTG. Characters have very little chance of combatting such units even under ideal situations and such elements are best left to the background and existing TTG.
[Yes, we see miracles happen in the novels....but the authors cheat]