I don't think it is. It doesn't appear in the up-coming AUK Calendar.
That's not to say ECE-ing it is impossible though. (I admire the mental strength of anyone ECE-ing post-PBP and of course pre-PBP is a logistical nightmare.)
It will appear as an event in the AUK Results later in the year because that's a convenient way to get a finishers list published and archived - so tying an ECE in at that point is not impossible.
Wouldn't it be treated the same way as ECE of any overseas BRM?
Am I right in thinking that if you did a 100km ECE on the PBP, that would take the total up to 1300km (thus a 12kmh minimum speed)? If true you would therefore have an extra 18hrs to do those 100km.
Yes.
If you're really feeling brave, if you added a 1300km ECE to end of PBP, you'd have 210hrs to do the 1300, but you'd still have to do the 1200 of PBP in the 90 hours. Almost 9 days! If you were to be in an 80hr group, you'd have 220 hours for the second 1300...
150km a day, sounds almost reasonable...
J