Elastomeric Paint as Duct Sound Dampener

Yes, but not much. And it depends on the character of the sound. High frequency sound is easier to dampen than lower frequencies.

Without having some idea of the frequency, it's hard to say. "60 cycle hum" is probably the most easily recognized frequency around. That's pretty low. Human hearing doesn't go below 20 Hz or above 16,000Hz, roughly.

So material density, thickness and stiffness can all attenuate sound to some degree.

You'd need to know the frequency and sound level at the duct.

Or, just put a very heavy coat of paint on and give it a listen.

The urethane based "spray-in bed liner" stuff for pick-up beds is really good for this. Most of it is viscoelastic and does a really good job knocking down sound due to sounding board effects. it is also another layer of insulation too. outgassing will be your issue.

You would probably have to build it up pretty thick to notice much of a difference.

You might want to try finding the absolute cheapest carpet you can find, cut to size, wrap it and tape it up, carpet side toward the duct. Carpet will suck up a lot of sound plus give you some insulating benefit.