I saw on the wiki that the GPIOs expose SPI and I2C. Could these be used to communicate with a digitally controlled decade box?
It would dampen signals with a voltage higher than 5V for example, and tell the CEE the scale information to display the correct value on the interface. It could also handle negative voltages with an inverter (and also tell the CEE the voltage is negative).
I was planning of doing this with an audio interface to use it as a scope, but the CEE looks much more appropriate and easier to tweak.