It should be possible to look up an address' balance at a given point in time. Fundamentally, the balance of an address is the sum of unspent outputs at that time. Thus, if the spent status of a set of outputs of an address can be timestamped, we can gather unspent ones based on that.
Issues
Since messages themselves have no timestamps, this would need to be based on the milestone. Also, query performance might be an issue unless we can index based in part on the spent timestamp/milestone.
Proposal
It should be possible to look up an address' balance at a given point in time. Fundamentally, the balance of an address is the sum of unspent outputs at that time. Thus, if the spent status of a set of outputs of an address can be timestamped, we can gather unspent ones based on that.
Issues
Since messages themselves have no timestamps, this would need to be based on the milestone. Also, query performance might be an issue unless we can index based in part on the spent timestamp/milestone.