Geezer
A reasonable question. From a Scottish perspective, the reason we usually turn down these requests is that you enter the wrong sector. From the south, all TMA inbound aircraft fly up the eastern side of the airway and speak to the Talla sector, with outbounds speaking to Galloway on the west. Were you to fly direct to DCS you would be entering the Galloway sector straight into the teeth of the outbounds. At busy times this would be somewhat of a pain to say the least as not only will the Galloway controller have to deal with the flight, he will have to coordinate something with the Talla controller as the flight is not now operating in accordance with any of the standing agreements between the two sectors.
At quiet times it might not be so much of a problem, though Manchester might well have their own issues with this particular request.
As for filing the route, I expect the plan would be rejected as an invalid routing but others may well correct me on that.