You can't. Nothing stops you from time-reversing the layer, though, to which you could then apply the effect and after pre-composing you can reverse it back. It's still going to be tricky, but may help.
When making a loop the best option is to find someplace in the middle of the shot where your actor is moving between the start and end of the loop. For example, if the actor was seated, then he stood, then he sat back down you would cut split the clip where the actor was standing. Now you move the first part of the clip to the end of the timeline and the second part of the clip to the beginning of the timeline so that your loop point is where the actor is standing. Now adjust the overlap so the actor is about in the same position in both clips and work out your transition. If you want to liquify to mask this transition then I'd pick a spot where the actor was as close as he could be to the same position, set the blend mode of the top layer to something like screen or multiply so I could see through the layer, then move back a few frames, set a liquify keyframe, then move to the point where I want to cut between the two frames and use liquify or other distortion tool to make the frames match as closely as possible. I would then set the out point at that frame for the top layer to hide the cut. Now trim your comp so the first and last frame are visible and render or pre-compose and time remap to set up your loop. If you time remap a pre-comp you need to go to the last time remapping keyframe, move back one frame, set a new keyframe, then delete the original last frame and add the expression loopOut() to time remapping and extend the out point to create your loop.
Aha. Sometimes the simplest is the best. Thanks!