It's a really ugly hack, but you can use a
zero width space mongolian vowel separator (sic!) to stop the asterisks from being interpreted.
On OS X you can use the
echo to generate the unicode character. It's not documented in the man page but BSD
echo on OS X supports
-e to enable interpretation of backslash escapes just like it's GNU counterpart. Copy and pasting the string from the Terminal seems to be problematic but you can pipe the output to
pbcopy to send it directly to the pasteboard.
If you wanted to write
*foobar* then you could get your patched string like this:
echo -en "*\xe1\xa0\x8efoobar\xe1\xa0\x8e*" | pbcopy
You can use the same workaround to frame your text with the underline character
_. The second space isn't strictly necessary but I included it for the sake of symmetry. Maybe someone feels like writing a greasemonkey script to automate this rather clunky process.