We've certainly had reports of successful bookings that breach the rules such as, if I recall correctly, -DXB//SIN-. I would try a few times on the phone with your first choice before giving up, though if you are trying to book this at T-355 I can see that booking the desired-but-invalid outbound would be a problem.

Failing that, would MCT work for you: that would be valid.

Or I've often wondered how multiple legs fit into this: could you start with INV-LHR-DXB in which case is the relevant (and therefore long-enough) measurement INV-DXB? You could pick any domestic departure but the APD saving at INV would pay for the detour.