This question asks: How large a body could a probe with ion engines land on and launch from? and made me think could an ion engine that is the payload be mounted in a way to assist in the launch?
- Designed an ion engine that could operate in non vacuum
- Found a way to power it
- Found a way to route the exhaust
All that while not adding any mass or drag (else it's pointless)
It could do something. However
- An ion engine has a thrust in the order of magnitude of 0.1N
- A rocket have a thrust in an order of magnitude of 10000000N
It would be pointless.
Sure, for a small body it might provide a greater fraction than a chemical rocket, but it will still be a rounding error.