I think we're not there yet. You'd need to add the Text Only Field, then make sure to Exclude them from the regular email campaign, then create a separate Text Only Email, then send Text Only people that email.
A bit of a pain, but important if your audience is more on the text side. I'd say only people who are deep into Linux/Unix are text only these days. I don't know the stats, but few people have turned off HTML in gmail or outlook.
So until Marketo adds that, the above is what you need to do. Perhaps you can post it as an idea?
Allow Text/HTML only option on form sign up and have Marketo automatically send just the Text version of an email if the lead selects Text Only.