Interesting topic...
If I understand correctly what you are basically aiming at is how do you build pages that will have good SEO for the keywords you are after but also appeal in a real world fashion to the surfer that you pull from the forum postings/sigs?
I think this is a bit of an issue if your pages are strictly formatted for SEO no matter where your traffic comes from..in the end isn't the point of having good SEO ranking to draw targeted traffic that you want to sell?
in my experience you have to walk that fine line with your text to make it appealing to both SE's and the surfer.
someone mentioned cloaking..sure I guess this is an option as well..but that can also be a risky venture..
I have used the option you brought up sending different traffic to different domains/pages based on SEO and real world ad verbiage that will sell...it does seem to work, but it is more work in the end.
I always try to find the happy medium and put my efforts into a single landing page that will appeal to the widest targeted audience..is it the most perfectly optimized SEO page?..not always..but that doesn't mean its not possible to get a fair amount of SE traffic to it.
I understand you are really working from a different perspective and more focused on the SEO aspect in this particular topic while still building a page that will still sell to a human..so I guess your best option may just be to send them to different pages.
The big question is, is it a crap shoot to some extent because you never really know where a certain percentage of traffic will end up coming from? It could be SE's on both counts...thats why I try to build "real world" pages and not build 100% for the SE's.
Dunno if that really helped you any but it's my
on the subject