I am attempting to allow a group of Force.com OneApp users to be able to Modify All on a custom object that is the child in a Master-Detail relationship with Account. The security access on our Account object is rather complex, and I'm not particularly excited about making my custom object Lookup to Account and try to figure out how to keep in sync everyone with Account access being able to see the custom object. That being said, it appears I can click the checkbox for Modify All access on the child of a master-detail object in the Profile page, but I get an error telling me that the OneApp user needs to also have Read All access to Accounts (which is not a checkbox that is available -- only Read). I am wondering if there is any way around this?
EDIT: To clarify, the goal here is to have a small group of people who are only concerned with the custom object to be able to edit all of them. They don't have any interest in the parent object (a different batch of people leverages most of the other objects), but there is no issue with giving them Read access to all the accounts.