zblesk
19
I'll point out this will not be sufficient; if your selection contains line breaks, the highlight won't work. You'd need to add them to beginnings and ends of every paragraph.
Incidentally, the other commands already present have some related issues.
On one hand, it behaves just as Mark would - it adds asterisks on beginning/end of the selection. However, the left pane renders it differently from the right one. (First line shows up bold.)