That's right. You can use BGRALayers for this, or simply have a collection of bitmaps.
If you draw on the same bitmap, the information behind the line is lost, so you cannot undo it with EraseLineAntialias. There is no way to know what was behind in this case.
Alternatively you can draw with Xor mode, but then you don't have antialiasing support, and the result is not always beautiful.