Annapolis YC has been using Appx U with SI mods since returning to racing during COVID.

You can see mods as you suggest in the upcoming NOODs SI‘s. 

U1 allows any race signal to be used.  We have been running races using starting penalties with App U by flying any flags prior to the sequence.  I would hesitate to fly flags during the sequence.  

I agree with Rick. IF a flag is used, it should be displayed at the 3 minute mark and come down at the start. This also helps with some sailors complaining they need a visual signal. And I would never use App U for a large keelboat class, unless you tell the fleet that the line set well before the warning signal     

Shannon re: " And I would never use App U for a large keelboat class, unless you tell the fleet that the line set well before the warning signal"

I'm not part of the AYC-RC, but I have competed my J/105 in several starts where the SI-modified Appx U has been used for all classes, the vast majority being keelboats >20 feet.  

It did take me a bit to get used to on the receiving end, but AYC-RC is doing a good job communicating via VHF what's happening .. and so I only once crossed the line 2 min's ahead of time :-)

Please note that in the Canadian edition of the RRS book, Appendix U is Direct Judging

For the rest of the world: 
Reference is to a US Sailing national prescription (Appendix U) for a 3 minute sound-based starting sequence commonly used in high school dinghy racing.
Signals identifiable by number and duration indicate time remaining at 3, 2:30, 2, 1:30, 1, 0:30, :20, :10, :05,4,3,2,1, start. 
Visual signals are optional, but times are taken from the start of the audible signal.

A touch late to the party here, but a question- The World Cup and other WS events display Class, starting penalty, and other (e.g. O) flags prior to the 5 minute signal. As the sound system is intended to keep Signal Boat personnel smaller in number with less to do at any given moment, does it make sense to 1) display Orange at Start minus 6 (minimum) minutes (with sound), Class and Starting Protocol (P, I, Z, U, or Black) (with the meanings associated in Race Signals (P) or RRS 30) at S - 5mins, then at 3 mins go with the Appendix U sequence? AP and 1st repeater drop 1 min prior to Appendix U Warning.

Thanks!