I think your best bet is using SAMPA.

http://www.phon.ucl.ac.uk/home/sampa/home.htm

Others will no doubt disagree. You can also use XML/XHTML entities (they are delimited with an ampersand-octothorpe and a semi-colon and contain a number [Unicode] in between). Depending on your audience's browser / OS configuration, this may or may not work. For example, schwa /@/ in SAMPA or /ə/ in Unicode (number is 601 in Unicode).

http://www.tei-c.org/Lite/U5-chars.html