I'd also like to chime in and say that you can do just that. I just flew on AA (and actually had a mostly great experience) on MCE due to OneWorld Sapphire.

Also, just as a note, if the AA system doesn't recognise your Sapphire/Emerald status for whatever reason (esp. if reservation was made before the status was upgraded), then it is very helpful to give them a call, and they'll sort it out.