How high?

I thought maybe they were around 20khz... that'd be doable with decent equipment, but I saw a blurb that indicated they might be 35 to 40khz, and you'd need an ultrasonic speaker for that. If the sound card can't handle the frequency, the old PC speaker interface might be able to. Or hook it up to a serial port and just vary the baud rate or data pattern to hit different frequencies...

The IR pulse frequency that the MX remote's can generate might be close to what your ultrasonic remote generates. You might be able to hook an emitter output from an MRF unit directly to an ultrasonic speaker, and hit different "tones" by holding down keys programmed for the correct frequency.

Maybe you could buy one of those voice diguisers and have it boost the frequency of your voice, and then you might be able to whistle tones to control your tv.

The possibilities are endless here...