Is this an acceptable way to add a light to a 3-way circuit?
You're describing a shared neutral, and some local codes allow it, some don't.  However, it's usually described with closely related circuits, like a single 12/3 wire feeding alternate receptacles in a kitchen's two 20A circuits.

In general, I think they're a bad idea because if circuits A and B share a neutral, you could potentially still have current in A's neutral with A turned off at the service panel.  In addition, if A and B are on different phases and there's a problem with the neutral, you could have 240V across all your devices.  Using a ganged breaker mitigates those risks somewhat.

How difficult would it be to replace the 14/3 between the switches with two 14/2 wires?  That would get neutral to the downstairs switch and wouldn't raise any eyebrows come inspection time.  If that would also be difficult, next step is to talk to an electrician that knows what's allowed in your area.