After much playing around with Vercel Queues, I've come to the conclusion that (unless I'm missing something here), Vercel Queues isn't really designed for inter application messaging - message delivery seems to be scoped to inside of a single application (even with the polling client I couldn't get this to work because of OIDC token application scoping). Because of this, I'm having to give up on Vercel Queues for now.

In my experience, loosely coupled inter-application messaging is one of the best usages of message queues. Please consider this scenario as an enhancement for your future backlog.