[jjl]

I have developed a commercial product that uses PC hardware and software to perform standards conversion, although in the case of the product I worked on, input was interlaced PAL, NTSC etc. video and output was progressive RGB.
The suggested approach ought to be feasible but the main issue to overcome is that, assuming the PC graphics card would be used for output, these cards really are not designed for outputting this type of video signal, the TV encoder devices fitted to some graphics cards are not programmable enough to allow 405 line output.
I think that the most promising approach would be to use one of the analogue RGB colour channels to output a composite 405 line video signal with this signal synthesized in software, hopefully this makes some sort of sense. Of course it would not be possible to display such a signal on a normal PC monitor so a 405 line set would need to be available when debugging.
As to performing interpolation etc. in software, I can state that this is most certainly possible and that more or less any PC built in the past 4 or 5 years has enough CPU horsepower and memory bandwidth to perform the computation required to capture modern standard definition video and convert it to 405 line output.

John