I submitted another related post about this (but it is still awaiting moderation), however it seems as though we've found the cause of our problem.
Whenever a GMail user has the Zoho CRM contextual gadget displayed at the bottom of a Zoho meeting invite email the email gets altered in some way that adds a gray background to the body and disables all links (e.g. the join meeting link).
Whenever we disable Zoho CRM integration for our Google Apps domain (thus removing the contextual gadget) the Zoho Meeting invite emails are able to be viewed correctly (no gray background and the links all work). So there appears to be some kind of bug in the Zoho CRM contextual gadget's integration whenever viewing a meeting invite from Zoho Meeting.
For now we are just going to disable the Zoho CRM integration with our Google Apps domain until this has been resolved.