The log manager's AbstractPropertyConfiguration$ModuleFinder uses ModuleIdentifier.create() rather than fromString(), which means it does not support specifying the slot via "module:slot" and can only use the main slot.
A work-around for this is to create a module-alias with some name and the slot 'main'. If you want to use example.loghandler:10, you could define example.loghandler.10:main as an alias to the former - module paths can be prefixes of others and you can use numeric segments, since they are just opaque identifiers. The upstream PR was merged, and will be in 1.5.6.
I had no real plans for a 1.5.6 any time soon, but if we need this soon I can release a new version whenever.
Verified with EAP 6.4.10.CP.CR1
Retroactively bulk-closing issues from released EAP 6.4 cummulative patches.