I bid on a filled 66 block at an auction, and dropped out against the other bidder who later told me he was going to send it to a shop in Florida that could 'sonic' clean out the filler. Never heard of that before.
I think this would be the only option I would consider if I was set on removing it. I would think there should be a frequency that would deteriorate the filler without damaging cast iron.
But with that said, I probably agree with the "install GOOD oil cooler and run it" option, using a good synthetic oil for it's higher temp tolerance.
