I believe the rules state that every player must keep score for the entire card. That can be done by any means including rock, paper, Udisc, PDGA live, or on toilet paper. A TD might not like that you are using UDisc but it's allowed. I can see where this might be an option in an area where your fully charge phone gets no signal. If, your entire card somehow has signal, and you don't all you have to do is show the TD the UDisc scorecard after the round. Once again, the TD may not like, but it's legal.