Well, in their defense I think it would be a not-insignificant rewrite of the feature to support HTML-formatted meeting requests and, though maybe this is my unsentimental side, doing that just to support hyperlinked images seems a little frivolous.

That said, you can try creating the signature in RTF format, though I doubt you'll get the result you're after. I don't think RTF supports hyperlinks on embedded images (even the images themselves are a little problematic). Remember: RTF means Rich TEXT format.

I think your best bet is to just use the hyperlinked text for the time being.