Your post got me wondering, given that there's going to be areas where air can't act as a link to hub airports why not run a plus train for long distance flights (akin to plus bus for trains).Going by the artists impressions the bridge will be a walk across a station concourse between two lifts/escalators.
Given the difficulty of providing HS2 service to Heathrow itself, and the small numbers you expect to use HS2 to get there, I'm curious about what you would suggest instead. The Midlands has no connecting flights to Heathrow whatsoever, so for an international destination that can't easily be accessed via another airport our choices are train to London and a slow tube, or tube then Hex, or driving or coach round the M25. Even with the complication of getting to Toton and changing at OOC, HS2 provides a far better route.
You could have a regional based map which determined the cost, but would allow travel from every station within that zone (as an example every station within Birmingham). You would have to check-in with an app on your phone or at a manned ticket office by a set time to guarantee your connection to your flight (as an example to enable you be on a train which gets you to the airport by 2 hours before your flight, but could be used for up to 30 hours beforehand, to enable an overnight stay, with the return leg needing to be completed 30 hours after arrival), the app would then track you and you could report delays as you go along. If you've checked in at a ticket office you'd then need to contact a guard to provide updates, although the system would be tracking any delays anyway.
By being able to use any station it would make it easier than regional flights, where that often requires travel to an airport and the extra costs that involves.
By having the 30 hour window it would allow you to use it for an overnight and maybe a bit of a look around somewhere, if anyone wants a longer break (say to look around London for a weekend) then advanced tickets would work well.
For many could work well, I'm aware that it's not going to suit everyone, but it would certainly save the airlines the need to try and serve areas which otherwise wouldn't be viable routes.