SQL-Server 2008 R2, update Microsoft.SqlServer.Types.dll
I suspect such a change would require a whole lot more than just swapping out a DLL to get the new definition. How would the engine know how to parse never mind process the function? You can't just add FORMAT() or DATETIMEFROMPARTS() to earlier versions by checking in a DLL either.

I think the only way you'll be able to get SqlGeography.STContains() is to upgrade to a version that has it (SQL Server 2012).