Even though we are using static samples, there are a couple of things that will help at least broaden the sound. It has been proven time and time again that, like words that are mispelled, the human brain will usually fill in the missing parts. I'm in no way saying that we will be able to have a perfect piano coming out of our keyboards here but, some very basic effects and proper eq should help in the live playing.
The first approach to using effects to any instrument when trying to duplicate is trying to replicate the sound that is missing. Since we can't have the warm overtones of the next string or strings, we can at least try to add some hints of noise that are pleasing.
Compression - only if it is very light. Remember compression can make loud notes soft and soft notes loud. All you are doing with compression is making the volume of all your notes about the same db. Very light compression will help you from sounding too wild and can actually give you resonance (if only perceived by the human ear) where it sounds pleasing.
Delay - Sound travels differently with actual resonating strings. If you are on one side of the stage, the sound will hit the audience at different times. Even though this is in the range of milliseconds, our ears do hear it. A real piano would be mic'ed but other mics on stage might pick up some of the pianos sound and it would bleed into the speakers at different times. With keyboards, we are usually plugged straight into the mixing board and this kills some of that bleed. A light touch of delay can help. Run say 5ms on the left channel and 150ms on the right with a fast (but not too fast) decay and that could help.
Chorus - again, very lightly. You will want to run with a longer sweep across the chorus spectrum but use that effect lightly and you can get a pleasing sound and maybe even ditch the idea of delay.
I would stay away from reverb for two main reasons. Where ever you play, that place will have it's own natural reverb. Usually, there is a final reverb applied to the overall mix of the band. If you are running one type of reverb and then combining that with another type of reverb from the mix and then fighting with the clubs natural reverb - well, disaster (in this case, I would choose a delay)
I'm in no way implying that this will solve any problems (hell, it may create more problems until you get used to how effects effect each of your sounds in different rooms and situations). I do know that when trying to make an artificial instrument sound more real, sometimes we have to try and make its sonic overtones match too.
Sometimes something as changing the pitch by 1 or 2 cents can make you stand out and, to the listeners, sounds really good. The problem we musicians have is - we are too critical of our own sound! How many times have you seen the bass player sweeping knobs and cursing his equipment while everyone else is screaming at how good he sounds?
