Ok, so you take a wakeup source on card insertion, and then the uevent code has support for releasing the source when an event has been sent. You'd presumably need some sort of token to ensure that the event sent corresponds to the wakeup source. Userspace then attempts another suspend, and calls the prepare() method for the mounting agent. Either that or the confirm() method (or both) verifies that the event queue is empty (presumably by making sure that there's no active fds in a set it's poll()ing on, or something) and then the suspend is attempted.
If an event occurs between confirm() being run but before the suspend is requested, you'd miss that. So the suspend needs to then fail, but that's ok if you've got a token from before you called prepare() and that token's been invalidated by another wakeup event in the intervening time. So it does sound like this could be made race-free, but it does involve adding some extra complexity to the uevent code. That's not an obviously bad idea in any case - it'd be a useful thing to have for avoiding races in the existing suspend path.
Now, how about the network case? It ought to be possible in much the same way (you'd deactivate the source when the packet has been read), but there's a pretty huge distance between an irq from a network device (where you have to activate the source just in case the packet is magic, which you don't yet know) and userspace, and it could get lost at various points in between - so you need to map every path that it could take.
This is all obviously possible, and I think your approach could certainly be made to work. But making it work well is going to involve a lot of code. Not a criticism, just an observation!
This is one of the reasons why a scheduler-based approach is more attractive in many ways - you avoid the kernel timeouts problem without having to add a huge amount of kernel complexity. It obviously does result in a bunch of additional problems on the userspace side, many of which could probably be mitigated with timer slack - but I'm obviously speaking in a purely hypothetical sense, since I haven't found the time to actually do the work necessary to make that workable. You're actually doing the work, so you get to ignore me here :)