Thanks for all the responses, everybody. As I was writing a reply last night, my computer kicked the bucket after nearly 6 years of faithful service. The good news is I might get a new MacBook out of this. The bad news is that it leaves me less money to spend on bike and guitar accessories.

I notice that despite my stating clearly that an Xtracycle is out of the question, two posters nonetheless suggested an Xtracycle as the way to go. I admire your steadfastness . I think Xtracycles are very nifty, but given my nomadic student lifestyle they're really not practical for the time being. Anyway, my Dahon folder is just about perfect for my needs... a disadvantage in this case is that I can't really mount proper panniers; an advantage is that I can raise my handlebars on the fly to get a more upright position if I bike with a guitar on my back.

I'm most interested in the backpack straps for my existing hard case, since they seem to do the job for a reasonable price. I agree with the other posters who are hesitant about transporting guitars in soft cases. Also, thanks to marqueemoon for pointing out visibility problems with a guitar on your back... I hadn't thought of that, and it's something to consider for what would be an hour-long ride through heavy traffic.

Given the need to replace my computer, I think I'm going to have to go the cheapest route, i.e. a really cheap guitar in a cheap backpack gig bag. Thanks for all the suggestions, though. I'll keep them in mind once my financial situation recovers!

And one day... an Xtracycle!

P.S. The Colorado Case Company's website (linked to in Praxis's post) is, bizarrely, blocked by China's net-censoring firewall. I just found that quite funny.