According to the SEO guys here, an email campaign looks less "spammy" if it has a plain-text version of it. I can't think of a case where we'll need to do a plain-text only campaign right now... but I guess you never know.

That's correct. Something like a textarea field directly within each Container Item that html can be placed in is what I was thinking of. It could appear only if that Item Type was chosen in the Item Type select menu.