I think you can do this in porgramming. First, you add the outside speakers as an additional external paging zone. The "all call" page becomes the paging to all four zones, to be used as you said for emergency paging.

Now, you need to recreate a paging call that includes zones 1-3, but not zone 4. If you set up a combined paging assignment in program 31-07 that assigns zones 1-3 to a common internal zone - that would give you the ability to page those three zones.

If you don't want paging coming through the phone speakers, you set up an internal paging zone for the combined paging that does not have any extensions assigned to it.

I haven't taken the time to prove the concept here - but I have done some similar stuff on Aspire, and I think this would work for you.