May I suggest adding 'rf' to the built-in dynamics? Beethoven and Brahms used it quite a lot, even if it is synonymous with 'rfz' (pedants might disagree). As did some other, lesser, composers (one of whose music I am currently engraving).
A far as I can see, it just needs adding
rfz = #(make-dynamic-script "rfz")
to dynamic-scripts-init.ly
There's an easy workaround of course - just put that line somewhere in your own music, but to me it seems as though it's common enough to build it into LilyPond.