I did the same thing with a bike. I call it a "silly singlespeed". mine is a cheap folder though, so the chain readily shifted to other cogs, and locked up. of course, it had this problem when it had a derailleur too...that's what Dahons are all about, self-shifting. i am in the process of fully singlifying it, and maybe then the fuggin chain will stay on.
I'd say watch your chainline when you manually shift--it may want to jump around. but if it works, it works!