I have decided to stick with the sleeves as they are. Mainly because the rest of the cardy is going to be so busy and patterned that I think people are unlikely to notice the plain st/st section under the arms. There'll be a point on either side where it will look a bit odd, but I think only people who are familiar with 'aran' style designs are actually going to notice (this has been borne out of experience today of trying to descibe what the 'problem' is to friendly non-knitters) - and the knitters will be too polite to comment ;)
I can live with it anyway, which is really all that matters, and the seam under the arms will look neater than it would on reverse st/st.
The sleeve also looks as though it will fit quite neatly into the front and back shaping too, which is a relief.