After experimenting with a few mirror effects using a second camera projected on a plane I've arrived at this:
This was achieved by using two camera's, one mirroring the movements of the other in relation to the mirrored world. Then I've used the camera depth and culling mask to overlay the main camera's image over the mirror camera's image.
Performance on my phone seems fine, and hits over 60 fps when only rendering the terrain. Adding the trees as seen in the .gif makes it drop to ~40 fps. Seeing as how we're making a point & click I'll be happy as long as we hit 30+ fps, so we're good.
Now to re-implement the world switch mechanics.