I'd like to be able to store 5 hours of music on my 256MB CF card, which works out to something like 110kb/s. (Surprisingly enough, I can tolerate this bitrate while exercising with my Etys.)

I've been using the LAME options "-h -V 9 --athlower -10 -Y", which seems to work pretty well and yields an average bitrate of 105kb/s. Is this pretty reasonable? Are there any (potentially non-free) MP3 encoders which will work better at bitrates like this?

Do not use VBR at such low bitrates! The VBR model at low bitrates is likely to have flaws which end up yielding less quality than ABR.
The LAME setting that will have the best results at these bitrates is probably going to be --(alt-)preset 110. Why not sacrifice a few minutes of music and go 128 kbps ABR? Or even better, --preset medium?
FhG encoders are rumoured to perform better than LAME at bitrates <128 kbps, so you might as well try the FhG encoder that comes with Adobe Audition, for example.