If I've understood your modified setup correctly, I'm surprised it doesn't work.
A PAR provides a scope around the packages and services exported by the bundles inside the PAR. This scope prevents the packages and services exported by the bundles inside the PAR from being (easily) accessed from outside the PAR. However, all packages and services exported by bundles outside the PAR should be available to bundles inside the PAR (unless they are "shadowed" by exports inside the PAR).
So by moving the Service interface bundle outside the PAR, bundles inside the PAR and Service implementation(s) outside the PAR should be able to share the same Service interface. Also, by having services exported by Service implementations outside the PAR, the bundles in the PAR should be able to see those services, which appears not to be happening in your case.
Please check that "shadowing" is not the problem. This would be the case if at least one bundle *inside* the PAR exported a service consisting of a Service implementation. In that case, the bundles inside the PAR would be unable to see any Service services from outside the PAR because scoping prefers services inside the PAR when shadowing occurs.
If you have a service outside the PAR which is not shadowed inside the PAR and is not visible to bundles inside the PAR, please would you raise an issue and attach as small an example of code as possible which reproduces the behaviour on dm Server 1.0.2.RELEASE or 2.0.0.M1. (If you are using an older version, you should upgrade first in case the problem has already been fixed.)