While overline BBCODE support would be nice to have we can live without it. There are two common ways of indicating a signal is active low in normal ASCII plaintext. nSIGNAL or /SIGNAL.
Its also already possible to resort to Mathjax.
\$\overline{SIGNAL}\$
gives: \$\overline{SIGNAL}\$.
Unfortunately, that's a mess for anyone with a browser that cant run the Mathjax javascript properly, and also doesn't render till the page is fully loaded, which is problematic for those on slow PCs or slow connections.