There are also the mid 90s era mtb cranks with a 5 bolt 94mm bcd. Those can get down into the 20s as far as tooth count. I think the newer, 4 bolt MTB cranks can also run rings that small. Of course these will be triple arms, but, aesthetics aside, you should be able to do what you're trying to do no problem.
Another option would to run threaded BB "euro" bmx cranks and get a 30t sprocket [instead of a chainring and cranks with a spyder].
What's the frame? And can you change the cog in the back to something bigger to use a bigger chainring/sprocket up front?
A little pricey, and definitely quality- but this site might give you and idea of the BCD's that you can get 30t on. Poke around-
http://www.homebrewedcomponents.com/store.php