The (top!) query generates an SQL statement like select ... from ... limit N, but MSSQL does not accept that. There the result set can be reduced like select top N from ....
Afaik, there is no way to change that in a custom 'backend', is it?
I don't know if adjusting the SQL generated from the expression is the best way to go, or allowing a library user to add new sqlosure expressions, so that I could write my own mssql-top.