If you build with SUBTARGET=mess you'll get a binary with MESS as the window title and the orange icon.
I added a macro statement for VSYNC
MCFG_UPD7220_VSYNC_CALLBACK(DEVWRITELINE("upd7220", upd7220_device, ext_sync_w)))
... but don't know where to put the actual IRQ code:
if(m_GDC_MODE_REGISTER & GDC_MODE_ENABLE_VSYNC_IRQ)
You should create a WRITE_LINE_MEMBER in the driver just like any other line. The example you have there which writes back into the 7220 device is for machines (mostly the 9801) which have multiple 7220s and one drives the sync for the other.