How can a I make a palette colour transparent?

[tmanni]

I don't see any fast method to do this. Rotating the image is imo not the solution because you have to re-rotate each symbol.
My way would be to create a new layer for each symbol (by selection and copy/paste in a new layer) and then move them to their new location.
A copy of the original layer would be used as background, where each symbol painted with white to clear them.