This is something that is mentioned quite often on the BA forum. Special meals will prevent you from being upgraded. If you want a chance of an OpUp then don't order a meal. Sure, you could say bring my meal from the lower cabin, but they don't like to do that as it makes you stand out.

I'm guessing it could apply to any airline. I recently heard a travel writer mention this as something not to do if you want a chance of an upgrade.