Check out the Upload API from TestFlight. You can create some sort of batch script you can execute at certain times. For example:
curl http://testflightapp.com/api/builds.json
-F file=@testflightapp.apk
-F api_token='your_api_token'
-F team_token='your_team_token'
-F notes='This build was uploaded via the upload API'
-F notify=True
-F distribution_lists='Internal, QA'