forcing delivery of plain text email when both plain text and html templates are present

I have a new user requirement that's causing a problem...

I have a user notification for which I build both plaintext and html templates and rails takes care of assembling a multipart/alternative msg that contains both parts. All good.

But, now some users with Blackberry's are complaining that they'd rather have just the text version. I thought their client would take care of displaying that, but perhaps they need to set something up and you can never really count on users to do so, so I'm looking at a new user pref to explicitly request plain text emails.

The problem: Since I have both types of templates, I cant figure out how to only send the 'text/plain' one.

I tried explicitly setting the content_type to this, but I still get the multipart/alternative result.

Does anyone know how to force the content_type format sent in this situation?

Note: I could set up a diff method in the mailer using a diff name where only the plain text template was present, but that would be kludgey and not very DRY.