Everything worked out well with my AA/AS SDFC yesterday, so here's a data point. I used AS miles to book a trip from RIC-ORD-SEA, with the first segment operated by AA and the second segment operated by AS.
I couldn't do anything on aa.com or via phone (the phone AAgent said I was not eligible for SDFC because I was using Alaska miles for the ticket). However, when I showed up to RIC, about an hour after my original flight departed, the agent at the ticket counter gave me a confirmed seat on the next RIC-ORD flight - no charge - and said I'd have to talk to AS to get the second leg rebooked. I said "no problem, I'll take my chances." At ORD, I walked over to the Alaska gate, told them I had missed my original connecting flight on AA, and they put me on the next ORD-SEA after paying a $50 fee.
Of course, if the later flights had been full I might have been SOL, but I was pleased with the way this worked out. As a Gold, I know I'm eligible for free standby but not free SDFC, so I suppose the AAgent at RIC did me a favor by confirming me on the first leg with no charge.