I'm attaching a build log showing the failure. I'll also attach a patch to the ebuild, which fixes it at ebuild level. Thanks to serkan on #gentoo-java for assistance.
Created attachment 267863 [details] brltty-javafail.log Build log.
Created attachment 267867 [details] brltty-4.2-java-ebuild.patch Fix the issue, at ebuild level.
This has been applied, thanks for the report.