Offering a free hosting for a month or whatever sounds great in theory, but as you said yourself, most of the people on this forum are web hosting companies themselves, so usage would be limited. You'd also run into an open invitation for a spammer or abuser to do whatever they wanted on the hosting account with limited to no repercussion --- the very heart of why Free Hosting never works.
As a web hosting company that currently sponsors HD (we actually provide the server that this forum operates on), we'd definitely be open to an arrangement, but there's a lot of factors that would need to be discussed prior to implementing the offer.
The monetary reward system does work, but it all depends on how much someone participates and provides valuable information. The moderators do a pretty great job with keeping the spammers away, and the forum members here do an excellent job of reporting any posts that get past a moderator (or alerting them to a potential issue).
Having spent more than a decade in forums and answering questions from people, I find myself answering the same questions time and time again - to be honest, I now skip answering some questions as they've been asked a thousand times - (how do I find clients, where should I advertise, should I offer cpanel, which is better - windows or linux, etc etc). So many of these have been beat to death, but few people take a couple of minutes to run a search in the forum (or google). That said, since we are all technically competitors, many of us don't want to give up any special secrets either. There are things that each of the hosts do on a surface level which are shared to the general public and clients such as software versions, backup software, general server tips - but there are also a lot of secrets that each of us have such as how to best configure MySQL so as not to overload a server, what hacker fighting tools do you use, specific network security questions, or where we receive the most clients from.
Artashes did have a program running here for quite some time regarding the top contributors that added value, but that requires a lot of man hours reading through each post and determining quality over quantity. A person with 25 posts can easily beat someone with 70 posts if those 70 posts were of low quality "yeah I agree" type posts. As such, the monetary program worked out pretty well.
I'm interested to see how others might suggest things though.