We are currently working on an USB sound card, which is capable of 192/24 playback with a DC straight output capable of delivering at least 4Vrms, which will be targeted directly at people who want to generate MPX in software.
If you have a 192/24 capable SPDIF output on your hardware, I can recommend you this little box:
http://www.space-audio.com/space_web_eng/space.html (please note that the USB socket is for power only)
It has a 2Vrms output level, and DC output is available as a CTO (customize to order) option for no extra fee.