You know it occurs to me that you're not the first person to request mono output on iPods and for the same reason. Perhaps you should provide some "Product Feedback" to Apple suggesting "switch to mono output" as a feature *(Enhancement Request).*
To be honest, I doubt if Apple will add such a feature to the Classic as they appear to have developed that model as far as they intend. However, if you suggest it now, then by the time you're looking for a replacement for your Classic, maybe newer models will have it.
Feedback (for the Classic) is at
http://www.apple.com/feedback/ipod.html.
I suggest you send it (rather than me) since I don't have your hearing problem and it would be better coming from you because you would be suggesting it from first hand experience. (... and Apple have been known to contact people as a result of feedback and forum postings.)
A while ago I supplied feedback with suggestions for Podcast features and when the iPod Touch came out, it had the very features I had suggested! I know what you're thinking - and you're right, they probably didn't add features purely at my request - others probably made the same suggestion as I did, but what do you have to lose? ( ... and yes, I did go out and buy a Touch, just for Podcasts!)
Phil