"To provide a mechanism for binding actions to expressions, MathML provides the maction element. This element accepts any number of sub-expressions as arguments and the type of action that should happen is controlled by the actiontype attribute." I think supporting this element is of low priority for now. MathJax gives access to this feature via non-standard LaTeX command and it is not really well implemented in Firefox.
I beg to differ on the "low priority" and in particular the justification by MathJax. Even though MathJax does not produce <maction> routinely, there are other systems that use maction for making formulae interactive; e.g. to elide brackets. For this it would be sufficient to just implement the behavior of the "selection" attribute, i.e. to display only the child whose number is indicated in it. And of course have it default to "1". This should be relatively simple to do and would be much more useful than the current behavior of displaying all children of <maction>.
Patches from bug 120058 and bug 120059 implements the maction@selection and maction@toggle attributes (With these patches, the tooltip and statusline actiontype will just display the first child). I guess that addresses Michael's use cases.
Marking as WONTFIX as maction is deprecated and not part of MathML Core.