Probably the simplest way to implement TeX or LaTeX would be to have a pair of delimiter characters in the text of certain nodes, e.g. $Latex Source$, and a button on the toolbar which when clicked starts a search for LaTeX source in the nodes of the mindmap. Text included inside the '$' characters would be extracted, fed to a latex compiler program, and replaced with an HTML image. Of course, the user would have to tell the system where to find the LaTeX modules, but that shouldn't be too difficult.
Also, there is the issue of making the search as fast as possible. For this, perhaps using a labelled graph approach would work? Each node would be labelled according to whether it or its subnodes had changed. If so, the search algorithm should then examine those subnodes recursively. If not, the nodes could be ignored, thus speeding the algorithm.