Erm, using your Charis SIL mods?
(Or editing font metrics for other custom fonts) Otherwise no. So far no one managed to find the place (.jar file(s) ) where those 3 line spacing values are hardcoded, just as no one found where the margin values are hardcoded for Kindle 3 and 4 (so as to make margin change permanent, not having to edit reader.pref file after someone plays with Aa menu options).