Maven can apparently be configured with the mostly undocumented files
~/.mavenrc (for current user) and
/etc/mavenrc (for all users).
In there, environment variables and command line options for the Java command can be configured.
With this, it is easy to set
JAVA_HOME just for the Maven command, which will lead to it running with the specified version.
Here's the content of that file:
Pros and Cons
To compile with Java 9, for example to check whether your project builds without errors, this is a low ceremony approach as it requires no other changes.
If you not only want to compile with Java 9 but also use Java 9 features, you still have to specify
<release>9</release> in your pom.
Note that this puts the pom into an awkward state where it is supposed to use JDK 9 but does not reference where it might come from (unlike the toolchain approach).
Another disadvantage is that it must be repeated for every developer on the project.