I came upon an extremely easy programmer that works with AVRdude and uses the COM port, the old ISP Adaptor ( http://elm-chan.org/works/avrx/a... ), and I decided to make a home-brew version of it including a digital isolator (ADuM1411: http://www.analog.com/en/prod/0,... ) and using a FT232RL USB-UART chip...
I post here the schematic, a board layout with gerbers and BoM, all made this morning... ;) I haven`t tested this, however it should be fully compatible with the regular serial port ISP adaptor, plus it has the ability to work with 3.3v or 5v targets at the change of a jumper, and the ISP side of the isolator can also be powered from either USB or the target. It sports both the standard 6-pins and 10-pins ISP headers, and has some LEDs for show... The whole thing is surface mount.
I would appreciate any commments, suggestions, and corrections, I hereby release this freely for anyone willing to make em... ;)