The usual recommendation is horticultural oil or soap spray. I don't know about the scent additives but I can't imagine they would hurt. I use Dr Bronners unscented baby soap which has no detergents. It's kind of expensive so if anybody knows a cheaper solution let me know.
I have been fighting a cottony cushion scale infestation for *years* now. Soap spray keeps them in check but they keep coming back. Like you, I live in a temperate area where CCS isn't native and have not brought any new trees in for a while I know that I simply haven't eradicated them all from my trees. I am going to switch to hort oil in the fall when it cools off a bit.
My biggest issue has been that it takes forever to spray using a 3-gal pump sprayer. It takes me an hour and a half to coat all my trees. I tried two different hose-end sprayers and they were complete junk. However, you can find horticultural oil bottles with built-in hose-end sprayers (Bonide brand I think, maybe Bayer). I read somewhere these built in sprayers are far superior to the re-useable sprayers. I am going to try this, and if it works well see if I can modify the single-use sprayer to be re-useable.
here's the link about the sprayers:
http://lawniac.com/battle-hose-end-sprayers-chapin-g362d-vs-ortho-dial-n-spray