I can report success, finally. I did try Bud's suggestion but was not able to move the pin. I was swinging a lump hammer, the biggest hammer I could still use with one hand, but the pin did not budge. There was a fair bit of bounce in the mast which was absorbing quite a bit of the force of each blow. I suspect that if the mast would have been on the ground and supported the pin probably would have moved.
Instead I ended up removing the fore-stay which finally gave proper access to the sheaves from the front of the mast. Then I cut straight through one of the sheaves and the stainless steel pin from top to bottom with a reciprocation saw. As I had previously drilled into one of the sheaves, as soon as it was cut through as well it came out in pieces and I could take out one after the other.
Now they are replaced with new Acetal sheaves and putting it back together was just a breeze.