run a forum without any database? Never go with forums without any DB... it scales very bad and will be slower in the timeline of your forum!
You wanted Simple Machines? Well, SMF is a fork of YaBBSE - YabbSE is a port of the original YaBB (Yet another Bulliten Board) from Perl to php/MySQL So your solution is YaBB - which BTW is what proboards runs under the hood... so if you want to talk scaleability it actually goes upward fairly well. http://www.yabbforum.com
you can link to an external mysql, if you know someone that is willing to do so... i think theres some security risk there.