"I'm not quite sure how you would modify a signal that's already pwm"...
It must be PWM for a reason, but that could be to minimise the resistor size as I outlined.
Regardless, if it's a circuit that generates the PWM duty cycle based on a resistor, that resistor could be potted so to speak (ie, replaced with a pot). Making a voltage-dependent resistor (to respond to electronic signal rather than a pot) can be done, but that adds complexity. (Is that a transconductance amplifier, or should I be embarrassed again?) And a duty-cycle to voltage converter would probably be the input (to the VDR) if required.
Many PWMs are set with an R-C combination, but others could be fixed in some program (PIC, controller, etc) or have some other voltage input (LDR, external, etc). That becomes a firmware hack exercise and is usually possible (even if it takes extreme work which is never economically justified) but for practical purposes, it probably isn't possible or practical. (How I loved computers that loaded their OS or peripheral call routines into RAM. A hackers dream....)
But I reckon I'll bow out of this. Yet again I have no detailed familiarity with the hardware concerned (crap!, I have zilch familiarity with it!) - but I intended only so provide some ideas or possibilities.
In my case, I'd return the display else threaten "misleading advertising" action. Then again, I have been silly enough to attempt workarounds, and they are probably still in my room(s) of unfinished project LOL!
Best of luck.
And davekra, thanks for your insights and links. You're the man, ... I mean... Poster.