Reviving an old thread to ask for clarification.
In training mode (which doesn't account for stale moves or DI obviously) I'm getting uthrow to about 30, maybe a djc uair, then regrab into dthrow chains.
With Fox DI'ing up and in (relative to dthrow's knockback, so in front of Lucas), it looks like there's still enough hitstun to regrab for a while, although it'll be tricky to space and time it right--but possible is possible. By the time Fox can escape on this DI, you can probably just djc uair once or twice and nail an usmash for the kill.
With Fox DI'ing the opposite way (down and behind Lucas), it looks like you can make the regrab with time to spare until such a time when you can just DACUS and kill Fox (or maybe throw in SH PK freeze to regrab or something?).
So what I'm saying is, if Lucas grabs Fox on FD, should Fox ever survive? If so, how?
EDIT: Looks like I wasn't quite DI'ing perfectly. Fox escapes Dthrow (DI away) to DACUS (tech) just barely at 100, which is early enough that Fox cannot be killed this way. So I guess DJC uair chains out of the uthrow chaingrab would be our best bet, assuming we're reasonably adept at those--Fox can probably do some combination of DI and SDI that escapes those before usmash kills, but maybe Lucas could do magnet combos or weak fairs to fill in that hole and possibly even finish with something else like fsmash or dsmash or bair.