Please note that "Friendly URLs" aren't actually needed. Search engines today (and for the past several years) have been able to index dynamic pages with parameters just fine. Switching to a different way to display a number won't do anything but add complexity to your forum configuration, damage the ranking of your site by forcing it to be re-indexed and treated as new pages, and add load to the server.
I agree with Motoko-san and Karl here. First of all, your boards aren't going to be indexed anyway, or if they are the indexed boards are useless. Second, putting your topics into URLs like /topic/12345.0 isn't any more friendly to search engines than the ?topic=12345.0 form because so many sites use dynamically generated URLs the search engines were forced to learn how to deal with this format.
I'm willing to concede that having URLs in the form simplemachines.org/community/Truly_search-engine_friendly_urls.htm might be some small advantage for SEs, or possibly the same with %20 instead of underlines, but your /topic/12345.0 isn't going to help you at all because there is no more or less information there than the other format.
It can be fun to code this stuff up though. I coded my whole cooking vanity website to use dynamic page generation, then turned all the links into what appear to be plain English descriptive titles and directory names, e.g. www.example.com/recipes/Chicken%Enchiladas.html
, which then gets turned into a ?recipe= format by my rewrite engine, and then my index.php generates the code on the fly. I did it this way because it was fun to code it up and get it working, not because there was any other particular advantage to do it.
I think the best way to get your site indexed is to get a sitemap and do whatever you can to encourage the SEs to visit your site. If you want to mess with your URLs to have fun there's no reason to not do it, but you're not going to accomplish anything that will increase your ranking.