You've got two great ideas there, both of which I'd buy:
I'm definately keen on the Carb Ice unit...
And also on a 'GA RadAlt'. At the low height and speeds of GA aircraft, you might get away with an ultrasonic transducer. You'd have to be very careful when calculating (and placarding) it's limits.
EG - not to be relied on. Not acurate at above 250'. Readings will be outside 'reasonable' limits when climbing or decending at rates above 300fpm below 50' and 150fpm above 100'.
If the unit could provide reasonably acurate readouts (+/- 50') while straight and level at altitudes of up to 1000' (way out of the range of a sonic device), it would be a good tool for bimbling about at low level on sunny evenings...
... or even as a strictly 'unreliable' ground proximity device for inadvertant IMC or cloudbreaks.