If they think the DTS key is the solution, why not just replace that with an "Ext Call" key that is a user speedcall that just dials 9. Then they can type the 10 digit number. Same amount of keypresses really.
This would be the best solution to this question.
In 1999 or so, we installed an SX2000 into a retirement community in Charleston, SC and they insisted on having 9-less dialing for their residents.
The problem that we encountered is that their DID ranges were all over the place and the local area was growing so rapidly that they were adding new local exchanges all the time. Each time they added something new, it would cause conflicts in the system.
The ARS digits dialed printed out was about 50 pages.
I swore that I'd never take part of a 9-less dial plan again.
Best of luck!