Just an update for those curious, and to provide some completely anecdotal data:

Today I purchased the flights I wanted for CPT-HDS. There were two seats available on *A (UA) miles, and two seats in L fare class for booking on flysaa.com. The price was right on the fence as to whether using UA miles were worth it, so I opted to pay the fare and my friend opted to pay with miles. As United allows cancellation within 24 hours of booking, we booked his first and then I booked mine after his was confirmed.

After it got confirmed, the number of award seats AND the number of seats in L booking class decreased to 1. After I booked my L fare class ticket, both went to zero.

Again, it's a very anecdotal experience, but that booking class/bucket (part of the 'Saver' bucket on flysaa.com) may be where they draw the *A awards from, at least on SA Express flights.