AVR programmer with serial to USB converter
AVR "serial" programming is in fact done over SPI. It's possible to bit-bang SPI using an FT232, which is why they're often used for this purpose.

There is an old trick to bit-bang SPI over the flow control lines of a real serial port. You might be able to achieve something similar with your USB converter.

http://www.ele.uva.es/~jesus/avrprg/powerless.html