Stephan,
No 'alternate method' and gauge input shouldn't be effected by your sim key assignments. Quoted from the 'Descentometer Operation.pdf' manual.
Note: In order to make the keyboard available for ICAO and custom altitude entry, this gauge
places the keyboard under its control when it sees a value is about to be entered. The gauge then
captures key presses as the data is input and the ENT key has been pressed.......
When you first turn it on (PWR/CLR button) it's waiting for the icao input and key presses go to the gauge until you press the ENT button. You can also press the PWR/CLR button to change your destination icao.
The same applies when you press the ALT button to change the target altitude.
Note: I downloaded and installed the gauge in FSX as I don't have FS2004 installed this time. Operation should be the same.
At that time [1909] the chief engineer was almost always the chief test pilot as well. That had the fortunate result of eliminating poor engineering early in aviation.