I had the same problem with the ISP of my addict. And this is how i solved:Sillin
, en Flickr
I know that you would need special machines to do the same, but i showed this, ´cos as you, my first idea was to cut a semi-circular groove and drill through the mast. But i realice that all the stress will go to the hole and you will need to put an external ring as backing, and i think that don´t look nice.
So the solution for me was to do an inner cone system in order to have more support surface. I don´t know what machinery you have acces to, but there are some ways to do this system in a less complicate way that i did. (It could be with two semicircular groove, a pair of seatclambs, the cone of an old stem and a tube with the inner diameter of your seatmast and a welded nut inside)
I´m not teaching, just brainstorming.
Good luck with this Project.
I really like all what is not just to buy fancy parts.