@Pjama: Oh, wait, re-reading your initial message carefully: you *do* actually see an "Alt" font being usable in the Kindle's reader? And you just can't seem to be able to change which font it points, to?
Because that'd be something entirely different
.
Check the contents of the fonts directory at the root of your Kindle, checking which font is actually physically there...
That might just be a case of the plugin failing to update it (for... whatever reason. I don't think I've ever used this feature once, so I can't really vouch for its behavior
).