I think that it is most likely to act as a hormone. As you say, this would do it a "primary" messenger (message generator) rather than "second" messenger (message conveyor). But that's as far as it's endogenous role(s) and action goes.

Obviously, exogenously administered dmt reaches the synapses and binds to receptors as a "pseudo-neurotransmitter just as all the other psychotropic drugs.