I've just realised that a fairly simple workaround to this Oanda plugin is to use the MT4/MT5 bridge and connect to a demo Oanda account in MetaTrader.

It's a little bit more faff as I need to also run this application in the background, and there could be some limitations vs. the direct API, but at least I can start forward testing my strategy and see how it gets on.

Anyway, I'm sure others have already utilised this approach but figured it was worth sharing on the off-chance it could help someone else.