And is the inclusion of overground services to Heathrow an indication that
Heathrow Connect will be an Oysterable service?

Paul Scott is correct - Heathrow Connect beyond H&H is not covered by
Oyster.

Having spent the morning flicking through the "official" media docs.
Things that caught my eye a

1) No mention of OEPs. At all. TfL have since told me they'll only
affect 0.04% of passengers which presumably is justification for their
low key coverage.
2) All existing NR PAYG arrangements are to operate under the "TfL
Rate" fares-wise, which makes sense.
3) There's some rather nice tables detailing the total traffic figures
for the top 10 stations of each London Franchise
4) The new "Oyster Coverage" map is indeed pretty epic. I've cut it
out and put it on the London Reconnections post.