I just did this, although I stood by for an earlier flight instead.

I was booked CMH-DTW-BOS for an 8 am flight. I wanted the 7:30 am BOS-ATL-CMH to put me over the PM threshold, but there wasn't availability in my fare class, so I SDCed to a 4pm flight via ATL instead. Then, I was able to SDS for the earlier flight.

(Actually, the Chisholm agent was able to confirm me on the first leg early, leaving me to standby for the later connection with a 5-hour layover in ATL. There are enough ATL-BOS flights on Mondays that I thought I would take my chances, and I was able to get a seat on the very first one. I realize that the 5-hour layover is against the SDC/SDS rules, so that was an exception, but it all worked out.)