I tried my C7 bayonet on my Type 97. It does fit but will not lock. It seems that a modification to the bayonet rather than the rifle could be done.
You are right. The lug on the 97 is longer,
so the the 2 latches on the bayo will not lock
(in other words they don't have a notch to lock into).
The bad part is that on the 97,
the distance between the lug and the bore axis
is smaller than the same distance on the AR by a good 1/8" or so.
This makes the bayo to sit with the rear part higher
and the bayo (handle and blade) are not parallel with the barrel.
The bayo looks like it droops. It's goofy.
A clamp-on collar with a lug at the right distance from the bore
can be imagined by someone,
but personally I would cut and mod an AR FSB (the chunck with the lug)
and put a bushing inside to fit the 97 barrel.