No. The previous part of the level gets removed. Try to go back and there's a wall in the way - with nothing behind it. You can see a bit of footage in my movie BCM161 at about 3:14.

: and FRoG blast the vent core ?
: I still want to find out what the "BIG prize" is.

I have no idea what they're on about. A shallow spot of googling reveals that "frog blast the vent core" is apparently a phrase to do with Marathon, but I didn't pursue it much further. Anyone care to elucidate/theorize?

: (Or do you have to use Fuel Rod Grunts, and lure them forward from earlier in
: the level?)

Impossible I think. They'd need to have the appropriate chasing behaviour; but I've never seen any sign of that.