I think there was talk that if the Harrow - Watford line went over to LU, LO would operate a Stratford - Willesden Junction via Primrose Hill service in order to provide a service for South Hampstead/Kilburn High Road.
The problem with running to Willesden is capacity alongside an enhanced Bakerloo. There've been a few proposals in the last few years, including a 2 tph Stratford - Queens Park service, but I see in the latest TfL track access the following update that appears to knock that on the head.
"The proposed LOROL passenger service from May 2011 is broadly consistent with the proposals contained within the CL [
Cross London] RUS. Where variations occur (e.g.
LOROL will not be operating a Queen’s Park-Stratford service), the parties (and TfL) consider that the proposed service represents a more efficient use of capacity and Network capability, in terms of managing the heaviest anticipated passenger flows."
In any case, the service they've come up with for May, of 8 tph peak, 6 tph offpeak plus freight between Stratford and Richmond/Clapham Jn via Camden Rd doesn't appear to have any space for additional services via Primrose Hill.
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Here's a (not as good quality as I'd taken it in - damn video compression!) video of the route
http://www.youtube.com/watch?v=qnMgGtiCio8
It's quite amusing about half way through to listen to the conversation where they're wondering where Euston's disappeared to.

I guess they must have walked straight past the posters, ignored the announcements etc, etc...