Wheee! I just found a writeup on how to include accents, breathing marks, and such to Greek character entities in HTML.
Here is the page with the details An example - to render a lowercase omicron with a rough breathing mark, simply write out the entity (e.g. ο) followed by the unicode value for the rough breathing mark (̔):
ὁ → ὁ
The other marks (including subscript iota) are in a table on the link I just gave.