My two cents:
While a BT/eva could indeed be done and even be interesting, it must be
noted that
'Mechs still use just conventional weapons, which would be completely unable
to harm Eva..
The power cable in the other hand would be a major weakness.. But to other
issues..
Eva is a life-form. This in itself makes things way complicated. It can, in
a proper
situation regrow its limbs. The AT-field in itself can be used as a weapon.
(Remember when Shinji(? don't remember clearly) hit AT-field full power on a
suburb?
The nearby houses were literally blown away. So much for tiny mechs..
Another issue, mostly in inserting evas to BT, not 'Mechs to Eva:
Where do you find pilots?
No second impact, remember? Thus no pilots..
And if second impact, there's too much trouble with angels
anyway..
IF you do some serious damage/take artistic liberties with Eva's structure
or storyline,
sure multiple crossovers can be done. But..
Oh well, I've said my piece.