@girish no that won't work, as the installation needs the central composer.json and also update it. It also loads more directories into the vendor directory, which is read-only in Cloudron. Did you come to some conclusion with your experiments?
Looks to me like the Mautic plugin concept doesn't work well with the Cloudron way of things.
Potential Scenarios:
Plugins should be installed with composer, a lot of directories and the central composer.json would need to be writable... not good.
Cloudron Mautic image would have some basic required plugins preinstalled, how to decide which? For me it would be (AWS)Mail, Custom Objects, Multi Domain plugins. Probably it would need to be different mailers. But this would increase maintenance, as plugins may have urgent updates outside of release cycles of Mautic.
Cloudron set up like WordPress (Dev) - like a preinstall but without continuous updates of core (because that would kill customizations). A little improvement, as the initial installation is fine and working (at least causing 50% of Mautic beginners trouble otherwise).
Cloudron self maintained on LAMP stack. Almost all the benefits of Cloudron lost.
Which other options did I miss?