If you create a new list called "Fake Users" with a top level, you will be able to accomplish in the exact same way as you have stated below, if I am understanding you correctly. Please see the attached.
As of today 5/6/2019, Anaplan does not allow you to transfer values of a line item with the dimensionality of Users to another of different dimensionality (say, Custom_Users), even with the use of a mapping (lookup) from Users to Custom_Users. The reverse is permissible: if the source is a line item of dimensionality Custom_Users, it can be brought into another with dimensionality Users using a mapping (lookup) from Custom_Users to Users. There's a distinction to be made, as sometimes, you do want the final working dimension to be a normal non-Users list (since that has a top level, etc.). In that case, the process would have been 1) input using Users-dimensioned line item, 2) transfer to Custom_Users, and 3) do calculations and reports using Custom_Users. Again, as of writing this is not permissible.
I offer two workarounds:
1) Run an import process that transfers data from the Users-dimensioned to the Custom_Users-dimensioned.
2) Use Custom Users as the dimensionality, applying a filter with dimensionality Custom_Users x Users (TRUE if Custom_Users = Users). This ensures a User can only see his/her corresponding Custom_Users list element.
I will be first to admit that neither are perfect, 1)'s disadvantage being dependence on a process to be run, and 2)'s disadvantage is requiring a Custom Users maintenance to Users, though this can be solved easily by using the same coding and doing a finditem. Let's hope some enhancement comes its way.