We have been working on using the REST API to import content into an email / template with mixed results (errors on import which we are working on now). Other apps allow a neat way of an HTTP/S import but Marketo doesn't seem to so looking for ideas/suggestions.
Have you tried using velocity scripting or the token insertion method that worked for Digesto?
Our APIs are pretty extensive. You should be able to do pretty much anything you want.
Marketo REST APIs: Asset Objects » Marketo Developers
Check out the resource above. You can create a new Email Template programmatically, put in whatever HTML you want, then create a new Email using that template and send it out, all programmatically.