Thanks for your advice! I'm hoping I don't have to do that, because people will be easily confused by having multiple Parcel items.
I was taking a look at the traffic with Fiddler, and I actually got it to work in Collector when a test item (also hosted on our own server) had it's Usage Limited to "ArcGIS_AndroidSDK". That layer was still unavailable (as intended) when I tried to access it from another AGOL account and add it to a map. So it seems to be a valid way to Limit Usage to Android mobile devices at least.
Edit: The iOS string is arcgisios.
As Doug mentions below, this still allows any application built with these SDKs to access the item, not just Collector.