Using SqlCmd variables in a database project

Does anyone know if it's possible to use SqlCmd Variables in a stored procedure within a database project?

For example I've added a $NewType SqlCmd variable to my database project. And set this variable to 'VARCHAR'.

But when I try and use this in a stored procedure in the project:

DECLARE @ClientName $(NewType) (100)

the project won't build, giving me a syntax error near @ClientName. I have SqlCmd mode turned on via VS SQL menu.

A similar example works in SQL Management Studio:

:setvar VARCHARTYPE "VARCHAR"
ALTER TABLE tempTable ADD newColumn $(VARCHARTYPE)(200);

Does anyone know why VS won't build the project and if there's a way around it?