Just adding another vote for the usefulness of an RCP function in Rhino. Would love to see this in V7 soon. I liked the idea of being able to use the Mirror command on a detail.
hahaha Me too! (Except I have not yet paid off my house, that is impressive!) Just adding another desire for this to come to fruition. In the meantime, are there are any good plugins that will do the job?
Not that I’m aware of. We’re just using clipping planes (to clip off stuff above the ceiling or HideInDetail) and showing backside of our panels with display mode that matches face side look with backside, so it kinda looks like RCP. Passable for what we do… 
That’s clever!
I found a workaround that seems to be working, I don’t see any issues yet. I basically imported my whole model as a linked block into another file, then mirrored the whole model across the Y axis. Then I made my ceiling plan in the layout.
It works great, except for to get it to show correctly I have to freeze some layers from my original model and whenever I update the model using the block manager these layers settings do not stick (think VISRETAIN from AutoCAD). Is there something I don’t know about that would help with this?
I had a second kid and the third one is on the way… 
I would gladly have a kid to finally see RCP in Rhino.
Please see Rhino 8 Feature: Reflected Ceiling Plan