I just added the ability for you to use HLSL shaders in the game (on screen and on events so far). I need to add on tiles and figure out how to apply this during battle (along with tint).
I'll let you use any HLSL shader I write, and you can get the Moonside shader from here. You'll have to wait until the next release to use it though, but put the file in the Effect folder when you do.. And of course make that folder as though it were a new resource folder.
When you mentioned that you need to enable the shader to battles, it reminded me that the background soundeffects on maps wouldn't play during the battles in the default RM2k3 version. Which is lame if there otherwise is ambience on the particular map and you'd want it to be there for the battles as well. Say, if there rained for example.
I don't know if it has been requested earlier, but would it be too much work to enable these for battles?