We have an interface defined like this in JIRA:

public interface com.atlassian.jira.web.pagebuilder.PageBuilderService extends com.atlassian.webresource.api.assembler.PageBuilderService

We register it like:

register.implementation(PROVIDED, com.atlassian.jira.web.pagebuilder.PageBuilderService.class, DefaultPageBuilderService.class);

This means that plugins can't access com.atlassian.webresource.api.assembler.PageBuilderService because it ain't registered. You can't register com.atlassian.webresource.api.assembler.PageBuilderService because someone has already used the "pageBuilderService" name and JIRA will fail to start. These two problems makes it impossible for people to get a handle on the cross product com.atlassian.webresource.api.assembler.PageBuilderService.