The introduction of an independent free-floating Scottish currency is no barrier at all to cross border trade - because the banks are already setup to support it and make money doing so. It's very straightforward.
Let's run through a scenario. As we have already learned Scotland runs balanced trade with the rest of the world and the trade deficit is therefore with the United Kingdom. So we'll take the case of a Shropshire based company exporting goods/services into Dumfries and Galloway.
The exporter could throw up their hands, refuse to accept Scottish money under any circumstances and demand payment in good old British pounds. The result of that would be the loss of the customer (probably to a Scottish based competitor or one that read and understood bank marketing leaflets), reduced turnover and profit and perhaps even layoffs amongst the staff. Only economists would think that is a rational response to the situation or at all likely.
In a real business, sales is everything and you do whatever you can to keep the customer. Exporters need to export. The first call would be to the firm's bank to see what they can do.
Here is the overview of the situation:
For simplicity I'm assuming that the customers and supplier both bank with RBS. That isn't at all necessary. This setup just cuts out the noise of clearing and makes the point very clear.
The Kirkcudbright branch operates primarily in Scottish Pounds and does its reserve operations with the Scottish Reserve Bank. The Telford branch operates primarily in British Pounds and does its reserve operations with the Bank of England. The customer's bank account is in Scottish pounds and the Exporters in British Pounds.
RBS plc owns both the bank branches and produces its own balance sheet in British pounds - because that's its functional currency. That means Scottish assets are translated into the functional currency equivalent at the reporting date according to the usual IFRS rules. In other words Scottish assets are reported in British pounds even though they are actually denominated in Scottish pounds.
So we take a sale of goods and services from the Exporter to the Importer. The exporter prices the invoice in Scottish Pounds because that's all the importer has. In this case 'no deal' unless the exporter does that. (Again that isn't necessary, the process works just as well in reverse).
Of course Scottish Pounds are no good in Telford. So they have a word with their bank in Telford, who is quite happy to purchase the invoice at the current exchange rate. So the exporter is credited with British pounds (which as we know the Telford branch just created ex nihilo) and the Bank takes the invoice.
This expands the balance sheet of the Telford bank branch - it has an invoice asset and a cash liability to the exporter, plus a bit of profit for itself.
Now because RBS has Scottish businesses it is quite happy to receive Scottish pounds. So it instructs the Importer to settle the invoice to the bank's own account at the Kirkcudbright branch - in the usual manner of any factored invoice. An intra group loan (which in banking circles is known as a 'deposit account') links that back to Telford and clears the invoice.
What has happened here is that the bank has executed a swap because it has a foot in both camps anyway. Therefore it can provide the swap service quite happily to those entities that don't have that structure available to them - for a fee of course.
The actual structure will be more complicated than this simple overview (the factoring operation is likely to be a separate subsidiary with multi-currency accounts for example), but the essence is the same.
From the point of view of RBS plc, it had liabilities in Kirkcudbright to the Importer and now it has slightly smaller liabilities in Telford to the exporter - plus an amount of profit for itself. The exporter produced and sold their goods and stays in business, the importer got their goods and can use them.
Win-Win all round. Another good day for dynamic currency systems.