Well, after some searching I found this:
https://www.hpacademy.com/forum/off-topic-discussion/show/how-do-i-control-dash-temp-guage-with-haltech-elite
I realize this is several years old now and may not be of much use to the OP. But I was trying to find a solution to this same issue of trying to control an OEM gauge with an aftermarket ECU and came across this post. Fortunately, I didn't give up after reading this thread! Turns out it's actually very easy to control OEM gauges using PWM, at least certain types. If the OEM gauge sending unit uses a varying resistance to ground, like an NTC coolant temp sensor, all you have to do is disconnect the wire from the OEM sending unit and connect it to a digital pulsed output on the ECU, assuming the ECU has it's own input for that parameter, like engine temp. Then it's just a matter of setting up a table with the duty cycle that will give you the right readings on the gauge with the ECU switching to ground. The varying PWM duty cycle to ground emulates the varying resistance to ground of the OEM sending unit. Best way to determine the proper duty cycles for the table is by simulating the relevant sensor input to the ECU so you can determine the duty cycles needed for the full range on the gauge. Takes a bit of trial and error, but all you really need are min and max and maybe somewhere in between. Really depends on how many hash marks are on the gauge.
I did this with for the OEM coolant temp and oil pressure gauges in my 1973 Datsun 240Z using a Haltech Elite 2500 and it works great! Eliminated redundant wiring and sending units.