There exist several different versions of BBC iPlayer. (Even if you don't care about the main iPlayer site, perhaps you might want to watch videos on BBC News one day, which uses the same code.)
Here is some (reformatted) Javascript from the BBC's site:
a = glow.embed.Flash.version(); f = ""; f = a.major>=10 ? "10player.swf" : a.major<8 ? "7player.swf" : a.major==9 && (a.minor>0 || a.release>=115) ? "9player.swf" : "player.swf";
It is clear that Flash 9.0.115 holds special significance; this was the version that introduced SWF Verification support. The 7player.swf file seems to have disappeared entirely - I get a "404 not found" error. That means there is still one version of the iPlayer which hopefully should not require implementing SWF Verification.
Navigating around the site with Gnash declaring itself as version 8 (with a patch applied to fix Glow compatibility), only radio programmes attempt to play using this version, and even then not the live streams. (No, gnash still doesn't play anything, don't panic.)
So it would seem to me that fixing Gnash's support for this SWF 8 iPlayer would be a good place to start.