I have developed a solution that works with mobile browser for the tracker.
https://www.routechoices.comFor now it is free.
For the organizer: You login, you create your event, add the map and calibrate it. add the competitors.
For the competitor: you open the tracker in a mobile browser
https://www.routechoices.com/tracker/ select who you are, enter a code given by the organizer and start streaming your GPS data.
No need for app, work android and iphone.
The source code is on github
https://github.com/rphlo/django-seurantaI am planing to add couple of feature like gpx upload/download and maybe private event but things should work as it is now.