Personally, I've never done it. But, I happened to have a 1.9 metal HG laying around, as well as a 1.5 11mm head kicking around too. The only problem I'd forsee is the oil return tube, which is missing on the 1.5. Basically, you'd hit the same snag if you were to use any hydraulic head gasket.
If it were me, and I had a few spare bucks, I'd weld some material onto the head right there, and then mill it.
[/img]http://i182.photobucket.com/albums/x82/tochtli83/DSC05594.jpg[/img]



Note: On this 1.5 head, some meathead bored out the main oil return line to mate up to a 1.6 head. I know that Jack at VWdiesel.net had a coupler that you can simply slip in there to do the same task.
As for welding and milling the head, it seems like zukgod1 did just that (not to the outside of the head, but filled a few holes, and drilled out the extra coolant holes that the 1.9 has. I think he had to weld his block too. He might have some insights. He has it shown in a link within this thread:
Building my mk2 1.6TD
If you do go this route, let us know how it turns out. You could be a pioneer.
Good luck