Hello Phil,
I saw systems where people tried setting up non-ISO currencies for specific purposes such as fixed exchange rate / financial hedging scenarios. Those setups often cause some trouble especially with automatic posting functionalities, currency revaluations and alike. For that reason I would suggest that you try getting a very clear picture why this non-ISO currency is required and whether no other way of bringing those transactions into AX is possible (e.g. overwriting/specificying the exchange rate in the ledger journal that overwrites the default AX Exchange rates). Please try to do an "extra round" (talk/discussion) with your colleagues and try to find out what and why they really Need this. If you can avoid it e.g. by using a Workaround try to avoid brining in another currency.
All the best,
Ludwig