I don't think this is exactly what your looking for, but more of a process that I take to do projections on the fly.
Yes, I use the ArcGIS online imagery (in a connected mobile environment). Yes, my data collection is using the same projection as the imagery. This allows me to create a mobile project and deploy it to devices.
Natively the ArcGIS Mobile application won't read in the X and Y (Long/Lat) coordinates into fields.
The problem with the Imgary is that the projection does not allow for Geometry Calculations in ArcMap. So what I do, is change the dataframe projection to something that will allow me to quickly calculate X and Y coordinates, it posts those changes to the database anyways and then close the mxd and do NOT save.
So that how I modify projections and still keep the mobile project working fine.
I hope this gives some insight as to how you may want to solve your problems.
Thanks,
Akhil P.