Some modern alternators have a Alternator light rather than a gauge.
How about putting a modified Temp gauge in place of the ammeter for oil pressure with a stock pressure sending unit and adding a light for the alt in the oil light hole.
OR
The light could also be a multi step voltage or Amp gauge with several LEDs behind the red filter. if the center LED is lit its the same as straight up on the alt gauge if the led to the right of center is lit its 10 Amps of charge going to the system if the led to the left is lit its 10 Amps draw on the battery or something like that. It could be externally shunt driven.
volt meter
Full Discharge
a lot of discharge
partial discharge
no charge or discharge
Partial Charge
A lot of charge
Full Charge