Even though you should be able to change the seals without removing the allen bolt, I would still try to get the bolt out anyways.
If you are going to the trouble to change the springs, oil, and seals anyways, it only makes a more complete job to completely dissemble and clean out all the parts. (who knows when the last time this was done -if ever-)
Plus, next time you need to get in there and do the job again, it'll be a lot easier the next go around.
If the easy outs don't do the job of getting the bolt out, very carefully drilling the head out will allow you dissemble so that you can get the internals out. Then you should be able to easily remove the rest of the bolt with a pair of vice grips holding the damper in you hands.
but that's just my opinion.
I Wolfie's method posted above works pretty good.
......I'd rather wake up in the middle of nowhere then in any city on earth.
*2014 VN900 Custom (16,200 miles and counting)