I reckon that if you cannot get a readable impression using rubbing techniques, then there is very little chance of getting a readable record.

The only other approach which might help would be to sidelight the markers with polarised light (light source passed through a sheet of polarising filter) from LH side, RH side, top (and if practical, bottom), and take photographs with very fine-grained film so that you get maximum resolution.

Then blow up the prints and see what you can find.

Hope these ideas help (no practical experience I'm afraid).

Cheers

david