|It is "truly" open drain.  It can't parasitically power up the part.
|Tried it myself to make sure.  With 5V between pin 3 RA4 and pin 5
|Vss, I read 0V on all other pins including Vdd.

Is there any specified behavior for pin RA4 while VDD is going away?
If RA4 is used to, e.g., switch a 9 volt battery which is powering
the PIC via a 78L05 or equivalent, is there any risk that the PIC
will decide to ground RA4 while it is shutting down?  Would using a
chip with a brownout-detect help in preventing such a thing, or is
that either ineffective or unnecessary?

Attachment converted: wonderland:WINMAIL.DAT 5 (????/----) (0000E0EA)