Hey wnorris, I don't know if you remember when we talked about this a while ago, but how have things progressed since then? You mentioned incorporating support for OpenID 2.0 providers which should fix the problem (For those using those providers of course), did you? I'm just wondering because I happen to be the developer for the official reCAPTCHA plugin now (You can find RC6 of version 2.8 here, I'm still waiting for approval to host it here).
I'm wondering if there's anything I can do on my side to make things work, though like you mentioned most plugins look inside the $_POST variable and it seems like only the plugin who gets to access it first, works. Let me know if you have any ideas, I'd be glad to 'fix' this plugin if there's a reasonable fix available.