If this is to be a kit style biz, or product, I'd definitely just use the standard ft232rl chip. Its well known, reliable, and requires little in real-estate and complexity. I'm sure it could be marketed as a more "advanced" kit with one SMD component. I'm sure if the product is good, it will still sell. Of course, you could also offer a version with all parts except the ft232rl already soldered in place, doing your own reflow or even hand solder of it.
I've soldered a few ft232rl's to boards, they are not that difficult (Much easier than the FPGA's I'v hand soldered).
I just see using a part that needs programming and extra components as a way to eliminate one nice clean component to be not worth the effort and "tech support calls" that may result. Of course it all depends on your target audience, but I'd think anyone that could build a usb part, and program it properly using DIP components, probably wouldn't have much issue with a low pin count SMD part either.
Good luck on your project, and make sure to let us know when you get something out the door!
Also, if you do end up finding a DIP usb to ttl chip, PLEASE let us know, I just don't know of anything myself.