MattH's idea would work very well for yACF riders. It's simple and effective.
However, it's a bit trickier for foreign LEL riders who may not be able to send SMS messages to a UK number (no roaming coverage, not all "text" message systems are compatible with GSM SMS, cost of international SMS, etc).
There seems to be separate problems here, with solutions that depend upon what we are trying to solve.
There are (at least) three system types we could be trying to design:-
1. Something simple and easy for yacf use only (maybe 20 riders? number plucked out of thin air). This potentially means that it could be set up manually for each rider.
2. Something that is capable of handling anyone who is interested in using it. More riders means some kind of automatic sign-up would be preferable.
3. Something that is used for ALL riders.
Then there are (at least) two main ways of handling the data input during the ride:-
1. It is the rider's responsibility. (e.g. they have to send a text message, or go to a WAP/internet page each time they want to update their position).
2. It is handled by the controllers as you get your brevet stamped.
Obviously these need quite different things setting up.
Personally, I'd like a system where the controllers handle it for everybody. This may be done using web forms or uploading a spreadsheet every so often - as long as there is mobile coverage. Using SMS could get expensive if you send a message for every one of the 500+ riders at each control. But that is extra work for people who are probably going to be quite busy already - and it is possible that not all controls will have mobile coverage. A computer and dongle would have to be provided at each location too.
Each rider having responsibility for their own would be simpler to manage, but with the problems of potentially expensive messages for overseas riders (though the actual costs will be minimal compared to the cost of getting here and riding - assuming they have phones that work in the UK). It would also be more difficult to generate real tracking (a table of times that each rider got to each control) without imposing a text format to be typed in physically tired and not 100% mentally alert (using a web form would make that easier, but not everyone knows how to use/has a wap/web browser on their phone). But it may be enough for viewers to enter the rider number and see a set of received text messages giving a narrative from the rider.
I can help on this (as I said, I can provide a standard UK number to send texts to that can be routed to an email or web parser, and in common with probably a lot of people here can also provide web hosting for it).