Is the Top gear performance available on Boot?
Also, I know Denny did the "River Song" / POB promo film with Ed Roach. What other promotion did he do for the album?
I also read once that he was planning to tour it with Christine McVie (or do some kind of concert with her), is this just rumour or was it a plan that never came to be?
I don't know about the possible participation of McVie, but a solo tour in the US to support "POB" was definitely planned. Supposedly, it was going to include a good hunk of the Beach Boys touring band (Billy Hinsche, and probably Ed Carter, Bobby Figueroa, etc.) There apparently were even a few rehearsals. My recollection is vague, but I think there are a few tapes in the Brother archives of rehearsals that either haven't been listened to yet and/or were labled as tour rehearsals but didn't have a tape inside. Something along those lines. I believe some tour dates were even booked, and one or a few even went on sale before the whole thing was cancelled. I think Jon Stebbins' "The Real Beach Boy" discusses the tour in as much detail as is available.
Apparently, the BB's did allow Dennis to promote POB at a few shows in 1977 by letting Dennis do a few songs from the album. I think even these instances were relatively rare. Again, this is mentioned in Jon Stebbins' book. I think they did "What's Wrong" and one or two other songs.