How to control segment values in iExpense allocations screen.

In iExpense the expense allocations fields are editable while submitting expense reports as the requirement was to allow users select segments other than the defauted ones from Expense template.

However we would like to restrict the segments displayed in the LOV according to the ledger. I mean a reqponsibility for US operating unit should display segments asigned to US ledger only. How can this be acheived. I know one solution can be defining security rules. But would like to know if there are any better solutions.

Thanks

User369214