We're running into the same issue here. Anyone any ideas how to solve this and/or whether support for reference sets in offline apps is on the (short term) roadmap of Mendix?
I don't know if it's of any help to you, but we've built pages in the offline profile where each attribute is in a navigation list item with a 'call nanoflow' on-click events. The nanoflow then retrieves the list and shows it on a page. In the next on-click the association is set.
The main challenge was that you can't sort lists in nanoflows, while the lists did need to show in a sorted fashion on the pages. So we had to add attributes to the entities that allowed us to use 'database' as datasource in the second page, which allowed for constraints and sorting.
Would this work for you?