Where would you like it and how would you like to specify it?

we have a conventions to cache this kind of artifacts under top parent's .external-resources.

at jenkins top level parent directory is ${WORKSPACE}

Yes, but how would you specify this so that the wrapper understands. An envar?

env var sounds reasonable.

-Dmaven.user.home=$myDir is undocumented but works already. Note that the current impl. is a bit picky about the order of command line args: it likes getting sysprops first.

we now have an env var called MVNW_VERBOSE... if you want to do something similar for the install feel free to send a PR @dantran ;-)

I scrutinized the source code and solved this by setting the MAVEN_USER_HOME environment variable. I am working on Azure:
set MAVEN_USER_HOME=D:\home\site\deployments\tools\maven

See https://www.lewuathe.com/maven-wrapper-home.html

I added this to the README docs now.