Hello.
I noticed that the libvidcap rgb32toi420 method has been actually re-used in several other projects including my own. So thanks!
Also I noticed that it shows some odd behavior, for instance a "diagonal green line on black" becomes a grey line with green splotches every so often. I think what it "should" look like would be a slightly blurry green line (mplayer's i420 does).
Anyway so this modified version fixes that, and seems to work all right:
https://gist.github.com/2502032
Cheers!
-roger-