Filler paint.

I'd use gyproc easi-fill. You need a filling knife, true, but it is easily sanded down later, you can make all the mistakes you want...

Use the knife to get some filler on there any old how, then faff about with it over the next hour or so using a sponge (the closed cell stuff is good because it doesn't absorb water). Maybe you have some pipe lagging left over? You could always pretend it was a car and use body filler. Works well enough but can get expensive :-)

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Try a variation on 'Swedish putty'.

Mix filler powder (plaster or Polyfilla)into slightly watered-down emulsion paint and brush on to wetted plaster. Build up in layers. It should work like an external high-build masonry paint.

Original Swedish putty was a stiff mix of oil based primer and plaster used as a wood filler paste but emulsion will be better on plaster.

Cic.

Sounds exactly what I need - I'll give it a try.