Currently hudson.plugins.parameterizedtrigger.BuildTriggerConfig and related classes is designed to assume that the downstream project is an AbstractProject, preventing it from triggering a WorkflowJob in schedule (see also perform, perform2, DescriptorImpl.doCheckProjects, etc.).
This could probably be relaxed by just checking for a Job & Queue.Task from which you can schedule a job with a little difficulty (check ParameterizedJob.getQuietPeriod and Job.isBuildable); or perhaps SCMTriggerItem whose scheduleBuild2 is more convenient.
Should work smoothly for TriggerBuilder. For BuildTrigger, can work to the extent that canDeclare is made to be false, so that the triggering is done explicitly rather than via DependencyDeclarer (and thus the DependencyGraph, which at least for now is restricted to AbstractProject).