Dominik:

currently, "scriptler" uses either jenkins classloader or TCCL;
https://github.com/jenkinsci/scriptler-plugin

do you think it would be feasible for "scriptler" also to support explicit classpath?

say, similar to how "dynamicparameter" plugin is doing this now:
https://github.com/Seitenbau/sb-jenkins-dynamicparameter

namely:
https://github.com/jenkinsci/dynamicparameter-plugin/blob/master/src/main/java/com/seitenbau/jenkins/plugins/dynamicparameter/util/JenkinsUtils.java#L97

1) provide capability to define "jar folder" location on jenkins
2) replicate this folder to all jenkins slaves
3) add these folders to the GroovyShell classpath

finally, since
"dynamicparameter" depends on "scriptler", you could probably
agree with authors of "dynamicparameter" to move this functionality
into the "scriptler" and make it a shared feature for all groovy script plugins?

thank you;

Andrei.