Hi Mark,
I have a similar issue-- I have a bad ear, and so I can only hear one side of a pair of headphones. Listening to things like Beatles albums (with hard-panned stereo parts) was driving me nuts! I could only hear, for example, the water sounds, piano, and backing vocals (or something like that) on "Yellow Submarine."
I started thinking that the only solution was to start converting all my MP3s (and burning new ones) in mono, for which iTunes does have an option. But that could literally take weeks...
But this solution was easy and cheap. I went to Radio Shack and got a little stereo-to-mono converter, about $6 I think. You can find them online as well. You plug that into your headphone output jack, and then plug your speakers or headphones into the other end. It takes both the left and right channel from your output and sends them both to a single wire of your speaker plug. Then you just have to figure out which side it's sending to.
Hope this helps for now. In the future, yes please, Apple, add this feature to iTunes!