I assumed that jreleaser can take the icon from project.icon and use that for the build. Why is it generating that image file in that directory?

The current implementation expects icon files to exist locally. IIRC project.icon may point to a local file instead of a remote URL.

But can't the icon file be generated in the build directory then if it needs to exist locally for the build to work?

Then the packager would need 2 input directories for source files: templateDirectory and downloaded assets. At the moment both directories are sourced from the same path. If the downloaded assets directory is realized as a separate entity the question now would be: does its path remain internal (that is, assume out/jreleaser/downloads/<distribution>/<packager>) or make it externally configurable?

The internal option would be the straightforward way and requires no changes to the DSL. Yet the pattern should be documented in case someone would like to use the download command to setup assets. Moreover, given that there will be 2 file sources, which one wins? I'd suppose the downloaded assets would override any matching assets found in templateDirectory.

WDYT?

I'm afraid you'll face similar issues with snapcraft as it also relies on a sandbox. sdkman and brew may serve as alternatives but they require splitting the portable archive in 2 distributions. Given that xpipe targets 3 platforms with 2 distros each you'll need 6 archives.

sdkman does not handle dependencies between candidates while brew does. sdkman supports all platforms while brew does not support Windows.

appImage might be another option but I think you may need 2 distros as well.