Spycygrl, I honsestly don't know if it's worth the trouble. SAWDLM gives 290 miles in Y, if the flight qualifies, BXNIST gives 272. So the total you can earn is 562 miles. Now I don't know if the retroactively credit, if you only enrol now.
Of course, you can use your 562 miles for rewards on SA ... but only 562 miles wont get you anywhere, I'm afraid. And you can't combine them with your UA miles. So unless you want to earn more miles on Miles&Smiles, the 562 miles are really not worth the trouble ...