Success!
Thanks for that forum link, it was very helpful. It seems the key is not to use the forelayer at all but draw entirely on the backlayer.
I implemented another bitmap as a buffer (as you suggested) to keep track of what has to be redrawn under the icon that is moving. This works great.
The code needs a lot of cleaning up, and there are still some issues that need to be addressed (such as: still doesn't work properly on PPC), but I am well on my way now.
I have attached my code in case you want to see what I have done.
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