Firstly, in all private pension provision, the contract exists between the client and the provider. This is true in the case of both workplace and individual pension provision. Many retired individuals have private pensions from several providers, and indeed from several different countries. The retirees themselves choosing perhaps to live in a country other than where the pension provider resides.
Whether the pension is a Defined Benefit (often called Final Salary Scheme) or Defined contributions (Individual Fund Scheme), the relationship of contributions, the management and ultimate delivery of the income from those contributions, is the responsibility of the pension provider.
Once the fund or income comes to be paid to the beneficiary, Pension providers are required to take into account country specific legislation related to the transfer of the results of the fund benefits to the individual's pension beneficiary. This off course includes taxes that must be paid. These can include basic and higher tax deductions, but may also include additional deductions if the total fund is above the UK Lifetime Allowance for example.
If Scotland at independence adopts the same legislative framework for pensions as presently exist within the UK, then there will be no material difference in the value of payments from the provider (wherever they may be in the world) to the client.
It off course must be assumed that over an extended period of time, the framework for pensions in Scotland will differ from the former UK model, as, over time, either or both the UK or Scotland adopts pension legislation changes to better suite economic and social conditions.
In the 200+ nations of the world, there is not a combination of pension provision, currency type, individuals nationality or place of choice of residence that has not already been encountered and successfully administered to provide pension payments.
There is no expectation that an Independent Scotland would encounter any administrative of legislative barriers that would prevent the delivery of existing or future private pension's payment to those choosing to live in Scotland.