Manchester does some concessions like that - not ENTCS passes (as I assume these W Yorks concession fares are????) because they confer free travel, but 'MCT' is a scholar's discount which is added just like a railcard for GM fares. The catch? These are not available from TVMs at all. Are Child CDRs or CDS (SDS where not available) all the same price as corresponding CNRs and CNSs? If so perhaps this is an option to be looked at - but Children can purchase Anytime tickets whereas people entitled to Concession tickets can not - or rather they could, but at the full rate! Furthermore I suspect if these are ENTCS (or whatever) holders, some will be offended by the large, reverse text "CHILD" printed on their tickets!
I've been over this a few times in my head and I do not see an easy way forward I'm afraid Ian. Perhaps modification of the message that pops up (but I bet it cannot be edited to show one thing when the cheapest valid ticket is a Concession and when it's not) doesn't say "there is a cheaper ticket available" and something more like "Are you entitled to a concession ticket?
However, two more things occur to me. The first is that the warning is indispensable because it stops people from buying Anytime tickets when Off-Peak tickets would be valid, and that is easy to do and the person has not a chance in hell of a refund form any of the staff around here if the make that mistake. The second thing to say is that although I appreciate people want to be able to access their Concession fare easily enough - it is still right there in the list of ticket types for them to choose, isn't it? And is it any more difficult without this inappropriate warning, for a person requiring a Concession fare to get that than, say, a railcard holder? I'm genuinely not sure. What are the complaints from people who need these fares that they are difficult to find on the machine? How sure are you that this isn't simply difficulty using the machines (slow, flashing, unresponsive screen which is also dim, all caps hard to read, strange font etc.)? Manchester (evidently!) get by not allowing loads of ticket types from their TVMs, Parkeon and otherwise. I appreciate you're doing the best with the resources given to you, Ian, but could it not come down to the fact that you have been given too few resources to solve this problem, given the bottom-end nature of the TVMs Northern have purchased? Not trying to poke (hope that doesn't read as if I am though), genuinely interested.
How is this avoided in Strathclyde? They too use individual ticket types don't they? Personally I think this really clutters up the database (like when TOCs run promotions by creating new ticket types such as "WEB ONLY CDR" or something, and should be avoided where possible!