FYI the same day I posted my questions above on converting/extending to a 6 or 12 month F in Beijing (which used to be very easy/reliable/'cheap' before the Olympics) I also sent out a rash of emails to the usual visa agents/consultants in Beijing, including a few I or friends have used successfully both before and after the Olympics. There seems to be a fairly tight consensus among them all on what is and isn't currently possible - I've summarized below.
Z/Residence Permit to F conversion - not possible in country. In one case I was forwarded to one agent in Guangzhou, who said he could do it for 7000 RMB, but I assume he just crosses into HK with it and applies like normal, so not worthwhile at all since you could have a nice HK trip for that price and get the visa!
L to F conversion - possible, only for a 6 month F, average cost around 5000 RMB
F to F extension - possible in either 6 or 12 months, multiple entry, cost ranges from about 2000-4000 RMB. All agents stated that it would be cheapest and most reliable for me to either fly in through HK and get a new F there or get a new one abroad before entering (but apparently in HK 1-2 month Fs are as easy as ever?). I assume that F-extensions are the easiest from their perspective of greasing the wheels, which you could argue either validates or arouses suspicion about their claims.
Just FYI, can't claim its tried and true, but as I said this is from a wide sample of agents in Beijing, some of which I know to be legit (as far as that word goes in this indurtry, at least!).
FINALLY - what none of them answered - when applying for a new visa (F, for example) while I still have a valid Residence Permit do I need a release letter from my last employer or not? I did not need one when I did the same in the summer of 2007 in HK (although I had it on hand, just in case), but there seems to be a lot of hearsay out there on this point. I guess the safe bet is just asking my old employer to forward me a release letter I can carry with me when I apply...
EDIT: Hope this post conforms to the forum/post rules, if not I'll post my actual results when I go through with whatever plan I settle on come May/June.