I haven't done it, but were I to undertake such a project, I would probably approach it thus:
- Make a cast of the front element (plaster? hard urethane? need to research materials).
- Apply polishing compound to the cast and place the damaged element on top. This should preserve the curvature of the element as long as even pressure is applied.
- Use a suction cup on a shaft and spin the element in the cast. If you want to be very precise about it, you would need to rig a brace over the cast to support the spinning shaft and prevent it from moving laterally.
- Move to progressively finer compounds until the element is polished.
The selection of the starting compound would depend on the extent of the damage (depth of scratches). You will, of course, lose any coating on the front element, and even with these precautions, you are likely to alter the optical formula slightly; whether this has an effect on photographs is a different question.
I've got a hideously scratched up I-50 that may serve to test this process at some point, but it's not on the high priority list right now.