In traditional vendor-driven networking world, you might be able to persuade your vendor to implement the knob (you think) you need in 3 years by making it a mandatory requirement for a $10M purchase order. In open-source world you implement the knob, write the unit tests, and submit a pull request.
This is exactly what Donatas Abraitis did when he figured out FRR cannot set additional attributes on BGP default routes and Russ White merged the patch into FRR master branch last week. Problem solved.
Obviously not everyone has the necessary development skills, and it might not make sense to Google-and-stumble your way through the obstacle maze to get the functionality you need, but you can always engage consulting companies like Igalia like a large ISP looking for high-performance NFV solutions did.