can you tile directly over plaster in a room being coverted into a shower stall
Always waterproof a shower pan and leave it filled with water for 24 hrs to check for leaks. If your installing a steam unit the complete shower must be waterproof. Speak to you local waterproof expert.
The answer is you should not do this, because even though you are putting tiles on the walls, this does not mean water will not penetrate to the wall board. It should not take long or cost that much more to do it correctly. You only need to remove the drywall or plaster board where the new tub or shower will be installed. Ideally the rest of the bathroom should have a moister resistant wall board installed but plaster is pretty durable so you should be ok with it in these areas.
For us we take everything down to the studs. This allows for a proper inspection of everything, and it allows access for plumbing, electrical, venting and duct work to be completed. If you are planning to install a shower door, T-paper holder or towel bars, this also allows the opportunity to install extra blocking (lumber) where these things will get bolted to the wall.
I know this morphed into a little more than the original question asked but I hope it has helped with your concerns.