Hi David,
So you use Marketo's My Tokens (http://developers.marketo.com/documentation/getting-started/) as placeholders in your HTML, and place the code/comment block into the My Token in Marketo. The Marketo compiler should strip out comments as it's preparing to send, and then insert the token values after that (at least it seemed to do that when we tested last).
i.e...
HTML:
<HTML>
{{my.outlookFix}}
</HTML>
My Token:
{{my.outlookFix}} = "<!-- Comment -->"
You would need to do a little research on the My Tokens if you haven't used them before, but they're useful for a lot of 'hacky' fixes. They're found in Marketing Activities, and can be placed on folders above programs, and on programs - but not on folders in programs. They only apply to items that are children of that folder, and can be overwritten by subfolders (scoped to those subfolders).
Regards,
Brody