Maven: Bind plugin execution to the execution of another plugin, not to a lifecycle phase - maven

Question

Note regarding the accepted answer: I accepted the answer because of strong circumstantial evidence. Nonetheless, this is circumstantial evidence, so take it with a grain of salt.
How can I have a plugin be triggered when the user runs a plugin goal, not a lifecycle phase? (This has been asked before, but the answer was to use a lifecycle phase.)
Case in point: I need release:branch to invoke regex-plugin to generate a branch with the current version as its name, minus the -SNAPSHOT suffix. This is what I have, which requires the developer to activate a profile and invoke the verify phase. I need the developer to simply invoke release:branch, which in turn should cause regex-plugin to run. In a bit of a marriage to Gitflow.
<profile>
<id>Release Branch</id>
<build>
<plugins>
<!-- On validate, compute the current version without -SNAPSHOT. -->
<!-- Put the result in a property. -->
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>build-helper-maven-plugin</artifactId>
<version>1.7</version>
<executions>
<execution>
<phase>validate</phase>
<goals>
<goal>regex-property</goal>
</goals>
<configuration>
<value>${project.version}</value>
<regex>^(.*)-SNAPSHOT$</regex>
<replacement>$1</replacement>
<name>project.unqualifiedVersion</name>
</configuration>
</execution>
</executions>
</plugin>
<!-- Also on validate, run the branch plugin, and use -->
<!-- the non-SNAPSHOT version thus computed in the branch name. -->
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-release-plugin</artifactId>
<version>2.3.2</version>
<executions>
<execution>
<phase>validate</phase>
<goals>
<goal>branch</goal>
</goals>
<configuration>
<branchName>release/${project.unqualifiedVersion}</branchName>
<updateWorkingCopyVersions>true</updateWorkingCopyVersions>
<updateBranchVersions>false</updateBranchVersions>
</configuration>
</execution>
</executions>
</plugin>
</plugins>
</build>
</profile>
The intent is for release:branch to move the current snapshot version (say, 1.0.5-SNAPSHOT) into a new branch, which should be named after the version but without the superfluous -SNAPSHOT suffix (1.0.5). The current branch should then take on a new snapshot version (1.1.0-SNAPSHOT, not 1.0.6-SNAPSHOT, because we want release 1.0.x to have room for hotfixes, so we reserve it for the branch) (I don't have the automatic computation of the next snapshot version figured out yet, so, if you run the Maven configuration above with validate, you will have to enter it at a prompt).

Answer 1

The evidence presented so far is rather circumstantial. I've done some research of my own, so it's best I share it here. The below are either more of the same "it's not possible", or the building blocks for alternatives.
jetspeed:mvn plugin --- run a specified sequence of plugins; the configuration to run can be varied via a system property; IDE integration concerns
Executing goals before plugin runs (StackOverflow) --- same question answered in the context of a custom Mojo
Make Mojo run other goals (StackOverflow) --- again, from the context of a custom Mojo
Configuring default Mojo executions --- Maven page describing how Mojos run - more circumstantial evidence
Triggering phases before goal execution (StackOverflow) --- roundabout solution to my problem, unfortunately answered in the negative
INTERESTING: Guide to Ant plugin development --- appealing to me, because, while it requires writing a custom plugin, it's all Ant + Maven configuration, no code to compile; presumably a lower barrier to entry
Creating a parallel lifecycle --- appealing approach, because I could fully control the contents of the lifecycle to where it would use Gitflow verbs; unclear how IDEs would integrate this; learning curve and adoption barrier concerns exist

Answer 2

No, you can't bind to a plugin to another plugin. Only to a phase.
In Maven-internal terms, a "Mojo" is the thing that does work. A "plugin" is a collection of mojos wrapped up so you can reference them from the POM. Mojos bind to phases only.
From the plugin development documentation:
Each Mojo specified inside a plugin descriptor must provide the following
...
phase ... Defines a default phase to bind a mojo execution to if the user does not explicitly set a phase in the POM. Note: This annotation will not automagically make a mojo run when the plugin declaration is added to the POM. It merely enables the user to omit the <phase> element from the surrounding <execution> element.
For further confirmation, see the source of MojoExecution (the JavaDoc for this class isn't helpful) and notice that there are two possible sources of execution enumerated:
An execution that originates from the direct invocation of a goal from the CLI
and
An execution that originates from a goal bound to a lifecycle phase
No other way to kick off an execution means you're out of luck (barring extraordinary measures like rolling your own plugin that combines the effects of the two plugins you want to link and then using your custom plugin).