It would be very useful to reference Order Extended Properties as a condition in the rules engine.  Is it possible to look at this?

+1 for this. Order Extended Properties aren't very helpful if they can't be used to run macros or build workflow conditions. This small feature would unlock a huge amount of potential. e.g. passing order tags or attributes to Linnworks for special handling, VIP customer details etc. In our instance, being able to now import Shopify order metafields into Order Extended Properties is a step forward but their implementation on the Linnworks side give this "new" feature very limited use.

Still very much in need of this feature. Using Macros, we can now get all sorts of data into Order Extended Properties but we can't do anything with the data without support in the Rules module.