An alternative formulation of the core idea behind FEP-0499 is to use raw HTTP messages, i.e. to specify the delivery inboxes as URL values of an HTTP header. But at that point, the endpoint becomes generic and can be used to amplify an HTTP POST to arbitrary HTTP resources/endpoints. Would that be better? Maybe, if you wanted to work with HTTP messages instead of ActivityPub activities. I’d be open to hearing your thoughts on this alternative take. See also: FEP-0499: Delivering to multiple inboxes with a multibox endpoint
Arguably, HTTP is a message protocol and not a framework. It is Layer 7, the application layer. If you want to virtualize another protocol on top of HTTP, you can do that, but you don’t need to – you can just use HTTP as-is, assuming you’re fine with the request-response model of messaging.