I've actually been looking at removing the quantity field, as it doesn't make sense in my shop (yes, it's a strange world).
I think you can probably get away with writing a migration to change the column type, changing a validation here or there, to disable only_integer, and then try it out.
You may have to change the OrderPopulator.
Warning: I'm a Spree Newbie, so don't trust what I say.
Hope this helps,