I've used Kenny's chopstick technique (edit: suggested by Kenny) many times on Elcon chargers, which have a thinner coating of what appears to be a similar potting material. I can attest that it works well!
Kenny, have you found a source for a replacement for this potting mix? I usually use a spray-on PCB lacquer, but it doesn't look anything like the same, and there may be advantage in Mitsubishi not noticing glaring changes, in the admittedly unlikely event that they take the cover off any module (rather than replacing the whole thing with a new module). Whatever was used, it would have to be rated for at least 400 V, not have an acid cure, etc. Something like the silicone that you can get in tubes for caulking guns, but runny.
I've had a half-hearted look at our local hardware stores, with no luck so far.
Edit 2: Plus, actual potting mix would look so much better than a clear coating, and it would mechanically protect the larger parts from vibration. at least to a degree. Also, we've been assuming that the potting is of the rubbery / silicone kind. Some are really hard, and have to be melted off. Do we know for sure that it's the rubbery kind?