This is a tough one... a good challenge.
I've got a couple of gauges on the a-pillar (Apex'i). They are indiglo and bright as hell. I can dim them using the standard dimmer for all the dash lights. The gauges dim just fine, but so do the rest of my other lights (i.e. HVAC, Radio, etc...).
My goal is to wire something (probably in-line) with the gauges to cut their brightness by half. Hence, full dash lighting but half gauge lighting.
I haven't had my coffee yet so I may be talking out of my a--, but I think this is possible.