Can I call SMO from pure T-SQL?
You'd use the sp_OA% stored procs for this. Or CLR since SQL Server 2005.

It can't be done via T-SQL directly because T-SQL is for data manipulation. So you have to use one of the 2 methods above. Unless you want to use xp_cmdshell to run a powershell script.

This also brings up one limitation of T-SQL: how to get an object definition to disk? And I guess one reason why you asked this. Again, CLR or sp_OA% will do it. 

One thing to note is that almost every method and property in SMO maps to a SQL command or query. So using T-SQL to call SMO which is effectively T-SQL is circular.

And to get the stored procedure definition you'd use OBJECT_DEFINITION... to get the other properties available in SMO you'd use OBJECT_PROPERTY or query the system objects.