I've been using mp3fs for ages but it seems the (excellent) replaygain tags in FLAC do NOT get translated like id3 tags do. This means some FLACs come out very loud relative to a lot of others / normalised mp3s.
From what I've read, replaygain is contained as a Vorbis comment fields (METADATA_BLOCK_VORBIS_COMMENT) of the FLAC header. There's some info here...
2008-10-12 12:34:13 UTC in mp3fs transcoding FUSE filesystem