Sorry it took a little while to get this going. I've been feeling totally crap the past month.
But now, at long last, we finally have LaTeX support on FES! Just enter your TeX like so:
[tex]\sqrt{69}[/tex]
And it comes out like so:

The git commit to create this tag is here, for reference:
https://github.com/theflatearthsociety/forum.tfes.org/commit/e555893047c2ba79dc39acbba9a524155fe8d07bIt uses a LaTeX-parsing service I've set up at mathtex.tfes.org, which is served via CloudFlare so that it caches the result of image generation for super-speedy load times.
Any questions, feel free to ask myself or pizaaplanet.