This is a (perhaps simpler) alternative to the idea I proposed in Ticket #3301 - I don't know which would be easier but I'll throw this one out also. How about a module that creates route sets. In the extensions configuration, each extension could optionally specify one (and only one) pre-defined route set, using a drop-down menu. The set chosen would affect outgoing calls placed by that extension. DISA configuration pages might also allow selection of a route set for use with each DISA.
The "Route Sets" configuration pages would simply list each and every existing route, with a checkbox next to each, and by default (when a new set is created) all the boxes would be checked. You would uncheck any routes that you don't want an extension (or group of extensions) to use, and whenever a call is placed from an extension that uses that route set, it would act as if any unchecked routes simply do not exist. Therefore you could alter trunk selection for a particular extension (or group of extensions, or DISA) by simply duplicating an existing route but changing the trunk selections, and allowing some extensions to use the original route while others to use the duplicated route with the modified trunk selections.
The only changes in the user interface would be the one new dropdown box on extension configuration pages (set by default to "no route set"), and then for the new "route sets" module you'd have a right side menu listing all existing route sets, plus an "Add New Route Set" option. When adding a new route set you'd simply give the "route set" a description and then uncheck boxes next to the routes you don't want to use, and then submit.
Optionally, when creating a new route, you could include a check box to "Enable this route in all existing route sets" and have it checked by default. If the user unchecked it, then it would not be enabled (at the time of route creation) in any existing route sets.
Just another possible approach that perhaps might be easier to code, inspired by a discussion with a reader of my how-to page at http://freepbx.org/support/documentation/howtos/how-to-give-a-particular-extension-different-or-restricted-trunk-access
