I'm using Activiti in my project with H2 for now, but our product is based on MSSQL.
So, I'm sure that I'll implement MSSQL support for Activiti soon, and send it for the team to include it in their releases. I will also post it somewhere so that other people interested in MSSQL support can use it until it gets into the release.
I don't think I'll make it for RC1, but I have a plan to do it sometime in November, hopefully.