Alternatively could you substitute the pedal spanner in Feanor's pic with a large set of mole grips done up bastard-tight on the pedal axle?
Sadly, that won't work.
Firstly, you won't fit mole-grips in the space: they are too wide, unless you dismantle the pedal to leave just the axle remaining.
Secondly, mole grips even on their tightest setting are no match for this job. They would just slip.
If I had to deal with this, and the 15mm flats were fully rounded, then I'd remove the crank, strip the pedal fully, leaving only the axle. Then we need to figure how to get huge torque on the axle, along with heat. Mole grips won't cut it. I'd be inclined to use an
ANGLE GRINDER to make flats on either side of the axle, so it could be gripped in a big well-secured engineering vice, and then use a big F-Off extenstion-tube on the crank, which I'd wrap in rags to prevent the tube from damaging the crank.