For our C users we use NXP's ISP serial download. In order for that to work we need to control 2 lines - one for RESET and one for ISP_BOOT. This is also the method Flash Magic uses for their loaders. We do produce a USB dongle based on an FTDI chip to do this, but it is a very low volume part for us, and therefore expensive. So we are always looking for alternatives.
We noticed an FT231X breakout board from SparkFun. It has the typical 6 pin arrangement from early Arduino boards, and has the other pins available to easily solderable pads. So we picked up a few, added the 1 wire from RTS output to CTS input and the 6 pin female header. And they work fine for our C download.
This board also works for our BASIC download/debug with only the addition of a 6 pin female connector. No other modifications.
To see other boards we have picked up over the years check the forum