An update on this issue:
While I can't directly eMail an ePub to my PlayBooks account, ePub attachments to eMails sent to me may (on my Android phone or tablet) be tapped and will then upload to my PlayBooks account where, in the PlayBooks app (on any device), the ePub may be tapped and will then download and open up in the app. (It takes longer to explain the process than to actually do it.
)
After I've retrieved the ePubs from the eMails and "deposited" them into PlayBooks (using my Android phone/tablet), I can then receive those eMails on my PC for eventual storage in my Calibre library.
As quite a number of my ePubs may be delivered to me in this fashion, it may streamline things and still let me use Google's Playbooks app (which I do like a lot for reading my eBooks) to manage the ePubs on my device and up in my PlayBooks cloud; probably better than manually downloading them where they get copied, willy-nilly, to whatever oddball folder some 3rd-party eBook app prefers.
The above is not a one-step solution but it does seem to move things in that direction.
I'd still entertain alternative suggestions from others' perspectives.
Thanks,
Barry