It can be done but is risky, you will need to ensure that only one pub-id appears on any one page at a time. e.g. you can't put two different pub-id's on the same page
yes it is risky but yes as adsenseplr said only put 1 pub-id on your page or forum otherwise all account get ban good luck
If you were to do something like this it would be extremely "black hat". I would change your sites settings so it could not be crawled, as google would likely ban the site once they saw it. Then, to get members, go onto numerous black hat forums to get members. Or, if you wish to rotate ads, wordpress forums (i believe bbpress) is well equipped for this!
Yes it is risky i agree with you guys, because adsense only want to place one publisher id at a single page