I do care. To quote from the OP.

What else should I be catering for? (Preferably with hints on how to detect and handle it :).

And from another post later in the thread:

Dealing with scalars that might be NVs, IVs, UVs and readonly is only the first part of the exercise. There are also RVs, tied scalars, blessed scalars etc. When I posted, I was hoping for insights into dealing with these also. I never expected to get hung up on dealing with readonly inputs.

I'm also curious if SvPVX() can yield you some pointers to interesting things, ...

I think I may be misunderstanding you? I look up SvPVX() and found

#define SvPVX(sv)  ((XPV*)  SvANY(sv))->xpv_pv
[download]

And SvANY() is

#define SvANY(sv)    (sv)->sv_any
[download]

Which doesn't clarify much?