This seems so obvious so I must me missing something.
In the Mirror command, I see Xaxis and Yaxis, for the two perpendicular planes, but which option is the Cplane itself? 
Yep, mirror in Z (over CPlane) has never been amongst the possibilities for the normal Mirror command.
I currently have a couple of scripts for this:
MirrorCPlaneZWHistory.py (634 Bytes)
MirrorWorldZWHistory.py (633 Bytes)
(as per the titles, if history recording is enabled, it is applied)
I also have an old macro that still works… but I would use the script.
! _SetRedrawOff
_Cplane _Rotate X 90
_Mirror _Pause
_Copy=yes 0 1,0
_Cplane _Undo
_SetRedrawOn
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That’s a super weird UI decision from McNeel…
But thanks for the scripts!
I think because Mirror has always been sorta classified like a 2D command - i.e. use a mirror line to represent a plane, assuming an implicit CPlane Z. I didn’t see an item in the Youtrack system for this, so I put one up there as an enhancement request.
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Hello - a macro for this is
! _Mirror _Pause _3Point 0 1,0,0 0,1,0
-Pascal
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Funny, that option has probably been there for years but I completely spaced it, I think sometimes my head is still stuck in the V2 days. 
I think it was added in V6, nowait V5. But… with IP being a thing, it seems like any plane or planar object should do for mirroring.
-Pascal
Yeah, that might be a good addition to save 2 clicks over the 3Point option.
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Why do you need it if you can achieve the same by mirroring about the X or Y axis in another viewport?
Hi John - that would only help when using the canned ‘big three’ world planes, or UPlanes.
-Pascal