Hm, good points. (Yeah, I'd actually assumed it only stole dollar signs with spaces either side, which would at least be less common; it seems it's not that well-behaved though!)
You could manually define each fraction in
conf/entities.local.conf (using unicode characters where possible):
1/2 ½
23/894 <sup>23</sup>⁄<sub>894</sub>
A bit labour-intensive though I guess. Would be better for a plugin to handle it.