You'll notice when exporting to phpBB, that for inline we add \inline, as in

[tex]\inline ... [/tex]

Otherwise is just

[tex] ... [/tex]

This was designed for an existing phpBB maths plugin.

For MathJax, can you use data from the 'pre-tags' option. For inline our editor creates

<code xml:lang="latex"> \frac{12}{34}</code>

and for normal equations

<pre xml:lang="latex"> \frac{12}{34}</pre>

We want to support MathJax, and will bring out a complete editor that uses it shortly. In the mean time I can create a new option that any tags you like. However, this would need to be for generic MathJax, not a phpBB specific variety of MathJax unless you can convince me this replaces the [tex] style above?