Thank you for the hints, guys. I came back to it this evening (I am stubborn ). First, attempted to use punch-and-hammer technique, hoping that vibrations will get it loose. No luck. After that, resorted to drilling. After (a) using wrong (dull and cheap) drill bit, (b) using wrong size of the drill bit (too small) and (c) drilling from the wrong end I finally did what was suggested here: 5/16 sharp HSS bit, from the side of the hex bolt head. It drilled out bolt head in a jiffy, and crankset finally came apart. Hurray!

It was not it, yet. 'granny washers' corroded and welded themselves to steel nuts and would not come off granny ring. After some tapping with hammer, all but one came off. That last one took a bit of soaking in PB-blaster and some very serious tapping.

All steel nuts and bolts are soaking in oxalic acid solution now, aluminum washers in degreaser. Tomorrow I will reassemble everything together, using one bolt from the spare sugino crankset that I happened to have at hand.

Thank you for your help again!!!