CREATE PROCEDURE sp_select @tabname VARCHAR(30)
DECLARE @sSQL VARCHAR(255)
SELECT @sSQL = "SELECT * FROM " + @tabname
EXECUTE( @sSQL )
You can combine multiple variables together in the EXECUTE statement if you want, so if your select statement goes over 255 characters then just use another variable as well. I believe the limitation on the 255 character variable length go away on Sybase 12.5.1, not tried so I cannot confirm this.
The thing to think about is the performance may drop, you wil have to do some testing.