In the end I made a custom service implementation to ios by following this example: http://docs.gluonhq.com/samples/gonative/
When doing this and after learning some Objective-C I noticed the BrowserService which already had been hinted by José Pereda in the comments to the question - the similarity to my own MyService.m Objective-C file.
After investigating this futher the check in https://bitbucket.org/gluon-oss/charm-down/src/11c36e187921/plugins/plugin-browser/ios/src/main/native/Browser.m?at=default&fileviewer=file-view-default ... if ([[UIApplication sharedApplication] canOpenURL:nsUrl]) - makes it unavailable to launch my custom URL Scheme.
By removing that if- statement in the BrowserService the BrowserService can launch another app that has registered a URL-Scheme. And it does it without launching safari/default browser can be mentioned.
So here is a Gluon app launching another Gluon app (that listens for LAUNCH_URL_KEY with RuntimeArgsService)

The correct format also given by José was: outputHello://<your text here>. So I could both leave it empty or adding (a URL to a file for instance) directly after the URL-Scheme part.
However a URL leading to an app's local storage can´t be read from another app due to iOS app Sandbox: What is Sandbox in ios , Can i Trans data between in one App to Another App in iPhone,if possible how
It should be possible to "open url with options" though as suggested here:
https://developer.apple.com/documentation/uikit/uiapplicationopenurloptionskey?language=objc.