It would depend on the resolution of the projector.
If it is a SVGA (800x600) or XGA (1024x768) projector then it probably will not work well. Either the Roku will refuse to work at the resolution of the projector or the projector will reduce the 720p image producing undesirable downscale artifacts.
The resolution of the projector should exceed 1280x720 such as a WXGA and have an HDMI port.
Another issue which might be implied by your question is if you can use the projector's own USB port to power the Roku Express instead of needing two power outlets for both the projector and the Roku power adapter. It isn't clear from what I can find that Epson can supply a full 1.0 amps from the projector USB port. The USB specification indicates they should supply at least half an amp but that isn't enough. You would need to contact Epson support to answer this question or be prepared to use the power adapter supplied with the Roku.
Last issue is the Roku Express remote. Keep in mind the remote for the Express is line of sight via infra-red signalling. If you plan to mount a projector on the ceiling with an Express stuck right next to it, that may work but for best results make sure you point the remote at the ceiling when you use it. Otherwise if you want a wireless remote then get the Streaming Stick+.