Ah Sven,
you have such a good memory of my attempts to turn my mast into a flame thrower, rocket launcher etc. Yep, that foam in my old E-27 cost me about 1 week of digging it out. Ended up using very long pieces of PVC to break it up little by little, and then a shop vac to suck out the parts to expose the next layers to mechanically break free. Really really sucked.
That all said, I am finishing up a total rudder recore, and poured my high density foam the other night -quite the experience, and I must say Foss Foam was of great help in guidance, but that's another story.
Back to this problem. As part of my above foam experience, I have done a bit of research on disolving polyurethane foams (wanted to disolve some of the foam core out of my rudder). Bottom line is forget it. There is a highly toxic and elusive chemical out there that I can't pin down that has some effect, but the bottom line is you probaby can't get it and don't want to use it even if you could get it, and you are going to have to mechanically remove this stuff. I am afraid you need to write off all your mast wires. They are going to be destroyed in this. Your initial thoughts of making a mechanical device to spin is probably your best bet. I would be tempted to start with something small e.g. 1 inch diameter pipe so you can control it, and make cuts into the end of the pipe so it will tear up what it spins against - I am thinking something like a piece of pipe with cuts down into it so it looks like the parapets/merlons on a castle tower. This would give you a toothy end that ought to rip up the foam. I believe you will find the wires are going to be history as there is no way to seperate them from the foam.
This is going to be a nasty job, and you need to cover all skin and wear a good respirator (not a paper think, a real one with pods). The dust you will produce is terrible - makes grinding fiberglass fun.
Good luck, and let us know what you did in the end.
-David
Independence 31
Emerald
p.s. for those interested in my recore rudder project, check here, and I will be adding the parts on pouring the foam over the next several nights, so check back for more info shortly:
http://home.comcast.net/~independence31/rudder/rudder-intro.html