I've finished looking at the underlying programming code.
The MTEXT codes that work (case sensitive) are:
\F and \f, font change.
\S, stacking; useful for super- and sub-scripting. Only the '^' sub-code works; no actual fractions are possible without drawing in a separate line. Examples: to produce superscript A, use \SA^;. To produce subscript B, use \S^B;. The final semicolon ends the stacking code.
\n, new line. (This isn't a 'real' MTEXT code, I think, but fairly standard computer code that gets interpreted as new line. I don't think it would be understood by AutoCAD if the drawing file was reopened there.)
As I noted before, other text decoration like underlining is not yet supported.