When I calculate the ABO, I use the Accrued Benefit. I still do the usual ancillary valuations I did in the PBO calcs. I project future service solely for the purpose of eligibility for future early retirement benefits, but still only reflecting the Accrued Benefit. I think this reflects the "grow in" rights you mentioned? I suspect we're concerned mostly with subsidized early retirement bens here; could be some other weird subsidized bens to consider, I suppose.
I think USA rules also require protection of grow in rights upon plan termination (there is an old 1984ish Rev. Ruling about it, I recall).
I've never worked with a plan that had Unpredictable Contingent Event Liabs (right term?) so I don't know about them.
Paragraph 18 of FAS87 does seem to describe a plan termination liability, but I'd bet most actuaries don't do a true plan termination ABO. For example, what if a plan termination would mean immediate cash-out offers for all participants? Would you calculate the ABO reflecting GATT rates pre- and post-ret? I suspect no ones does that. Could lead to large "Charges to Equity"?