If there was an app that would only be used at controls so it registered your location and time when you pressed a button that would match the validation of the Brevet card. Would be simpler than tracking and would be much less prone to failures and would not even require the phone to be on for most of the ride.
These lazy days are making me think.....
The Dutch have trailed a system where by when you got the control, as well as getting a stamp, you gave the controller your brevet card number, and they entered it into some system on a tablet, which tracked who was where and when.
Other options would be to include a simple barcode on the card, scan it at the control, boom, done.
The problem with a barcode or QR code is that it would require individually-printed brevet cards per person (or at least pre-printed name/code labels - which wouldn't support EOL entries of course) and although this isn't at all difficult to achieve technically I'm not sure anyone has the will to go down that route. In practice handing out personalised cards at the start of a big event is a much bigger job than just handing out generic ones and getting riders to write their name on. Plus barcodes aren't very weatherproof (don't know if QR codes are more robust?)
Regarding the Dutch method you describe, AUK's existing online recording system has a latent ability to support that, thus providing "person X was last seen at control Y at time hh:mm" functionality - that is, at any event control where the controller has the time and inclination to do it, and for any rider who has consented to be tracked in this way. It would take very little effort to activate that latent ability (and now is such a good time for infrastructure changes!) but again I'm not sure if the market is there - perhaps this questionaire will reveal something.
My personal experience as a sole controller 'live finishing' an event (a late PBP qualifying 600 where the results had to be processed quickly) sitting in a car park with a smartphone logged into the online Start List, with a button against each name. Just click each rider as they present their card, the server automatically adds the elapsed finish time and marks the rider as 'finished' (which appears online as 'provisional'). **
That all worked very well but the practical problem remained of writing the actual (not elapsed) time in the finish box of the BRM card - if a group of several arrived together this was too much for a single controller to manage. I prioritised the smartphone and tidied the cards later, but since it was a BRM and the physical cards still had to be processed in the usual way and as quickly as possible because of PBP qually, having to reverse-engineer the times back onto the cards made for a very late night for me after a long controlling stint.
With two controllers it would have been fine - and much the same applies for doing a similar thing at intermediate controls. Two controllers is a luxury many organisers don't have, and if they do then in all probability one of them will be prioritising the tea-urn or the sandwich-spreader, and they wouldn't both be checking people through.
** this facility does exist, for any organiser to use - however it's never been properly rolled out and has been stuck in 'beta' since I wrote it in 2011. The same facility is easily extensible to intermediate control tracking.