I have had success w/ claiming a seat in C or F by getting to the airport early and getting on the VDB list first.

That said:
1) you have to ask for a seat in C/F - this will be a portion of the compensation for your VDB (FA's won't offer it)
2) its 50/50 at best (but WAY better odds than your wistful op-up dreams)
3) you have to check for availability on the later flights