One thing I'm not sure about is whether you could actually buy said ticket
at the London end - outboundary Travelcards are kinda intended for people
travelling into London, and I believe they're excluded from the normal rules
whereby a ticket office is supposed to sell any ticket a punter asks for (or
maybe that only applies to outboundary Day Travelcards?).

Shouldn't be a problem - I used to buy a Basingstoke-Zones weekly/
monthly season from Ealing Broadway, as I lived in W5 and worked in
Hampshire. I never had a problem buying this ticket at EBY.

M