It’s unclear for your case. It’s largely dependent on what is supported by what you’re using that is out of your control. My expectation is that the release task is added by org.some.otherplugin and that you can’t change its code.
Also, the code shown doesn’t really indicate what how MyCustomClass and someReleaseFunction() are interacting with the release task. Typically you would be setting properties on that release task that would be used when it executes. The type of the properties on the release task would determine what you can do with a Property<String> from your extension. If your someReleaseFunction() is actually doing work, not configuring the release task, then there’s a fundamental configuration time vs. execution time issue with your code that needs to be resolved first.