SQL server isn't supported probably (and I speak with no authority) because of a few reasons:
1.) It costs money to acquire legally
2.) It doesn't necessarily adhere to the SQL standard
3.) It's Microsoft
I kid with the 3rd point. But honestly, I use MySQL and PostgreSQL for my sites, and I have yet to be upset. Even on an IIS server, you can run Postgre or MySQL
If you know PHP, and you know MSSQL very well, then I'd suggest you take a crack at verifying that everything in the RC codebase is MSSQL compliant. If it is, tell the devs and let them know that they can add MSSQL support. If not, perhaps you could help add another DBMS to the system.