Is it bad etiquette to contact a named reviewer? A named reviewer has made some suggestions, which I'm exploring, and I'd like to ask for input on a particular point. I have exchanged emails with this person before asking for input on the same work. Would it be appropriate to email directly to ask?
If the reviewer has given you their name, then that is their choice. In my experience journals give the reviewers the option of anonymity. If they've chosen to reveal their name then I think there is no issue with you contacting them directly. Especially if you've discussed the work with them previously. 

When you resubmit the paper, with the changes, perhaps you could then include this persons name in the list of people not suitable for reviewing the paper (Assuming that this journal asks you to submit names of suitable, and not suitable, reviewers).