U can be a good indicator, but remember that buckets are nested. So while when U is available, there is likely C inventory, the reverse is not always true.

C usually has more seats available than U, so the AA.com approach will not find all the seats.

Also a note, even ExpertFlyer doesn't always show all of the available seats. There are many times where I have seen C0 but the EXP desk was able to confirm them when calling.