I'm not really sure something like that can be taken into account alongside dates available for venues, and other guests availability etc.
It's unfortunate that it has fallen that way, but I don't think it's something that will be high on the priority list for timings.
I know, but I'm just saying that that is one of the factors that might affect the number of attendees. NOT that Barrowman would otherwise have attended the hub, but that there are fans of TW who would normally go to a TW convention whether Barrowman was there or not, but who are also fans of Barrowman and would not want to miss his concerts to do so.
Actually the reason Im going to the Hub is the JB tour! I was so happy to see that I could fly over to the UK and do both the Hub and two of the concerts. I don't think I would (or could have afford) to have gone over just for the con itself this time. It's perfect when two things I love is around the same time!