How can I add gradle project dependencies without modifying settings.gradle

Question

Background

(Please keep in mind I've simplified the problem for purposes of discussion here)

I've got a set of applications and dependent libraries, something like this (each with a src/ directory and build.gradle):
```
appa/
appb/
libx/
liby/
libz/
```

In build.gradle, the dependencies are currently declared like this:

appa/build.gradle:
   compile "com.asdf:libx:1.0"
   compile "com.asdf:liby:1.0"
appb/build.gradle:
   compile "com.asdf:liby:1.0"
liby/build.gradle:
   compile "com.asdf:libz:1.0"

What problem am I trying to solve

Say I'm working on appa, and I need to make changes to libx. I need to do multiple steps:
1. Pull libx from source control and make changes locally
2. Rebuild and push changes to some repo (not prod!)
3. Rebuild appa (pulling the recently updated libx from repo)
If my testing reveals a bug in libx, I've got to repeat that over again.
This is super-annoying when working in IDEs like Eclipse, where even though my projects are logically using other projects, I've got to still use the artifacts as dependencies.
Wouldn't it be great if I can just pull the project locally, and projects that logically depend on it will automatically use the source project instead of artifact for building?

What I've done so far

I've written a small gradle plugin (referenced in each project's build.gradle) that identifies com.asdf dependencies, and uses dependency substitution to replace the artifact dependency with a project dependency if that project exists locally.

configurations.all {
    resolutionStrategy.dependencySubstitution {
        all { DependencySubstitution dependency ->
            if (dependency.requested instanceof ModuleComponentSelector && dependency.requested.group == 'com.asdf') {
                def targetProject = findProject(":${dependency.requested.module}")
                if (targetProject != null) {
                    dependency.useTarget targetProject
                }
            }
        }
    }
}

Yay! With a few modifications to settings.gradle (see below), I've accomplished my goal... Except...

Where I'm stuck

I need to modify settings.gradle to include lines like this for every dependency (otherwise findProject doesn't resolve the dependent project during build):
```
include ':libx'
project(':libx').projectDir = new File(settingsDir, '../libx')
```
While it's possible to go through all the settings.gradle files and do this (I've done it for a handful as my proof-of-concept), it's ugly, repetitive, and is logically the same information that is being passed to compile arguments for the build.gradle dependencies.
- It's also error-prone when someone adds a new dependency but doesn't update settings.gradle, or introduces a typo between them.
I've also tried making settings.gradle just define projects for all directories it finds at that level, but then building any project turns into a mega-build of all projects.
(I've tried several other things, but my question is getting long in the tooth already)

My question

What's a better way to do this, without duplicating information between settings.gradle and build.gradle? I want to make it so adding new dependencies is still just as easy as adding the compile reference in build.gradle, without touching settings.gradle...

I'm still rather new to groovy/gradle, so maybe I'm missing something that's obvious to the more experienced gradle master?