Thanks everyone. The display options XML file worked a treat. I do know to use a span; as I said in my original post, I was trying anything and everything to get it to work, you just happened to see the one with the div.
I also know the state of reader support of CSS is … suboptimal. I only tried the others just to see if I could identify a general problem. Since I only really care about iBooks and maybe Marvin (this is for me, not publication), this was exactly what I needed. Thanks again.
(And I have asked the mods to move the thread.)