Well, your perspective here certainly seems to confirm the question raised by the OP - that organizations like DI, AiG, and the like are criticized for not publishing in peer reviewed sources… but when they try to do so, they are dismissed a priori because of their perspective.
You may be absolutely right - The park service may well have decided that, given this man’s perspective and goals, he, in their opinion, was not doing “real science.”
But if so, this simply affirms the original question asked… that “scientists working at Discovery Institute, AiG, etc. have a legitimate complaint that their research is unfairly discriminated against.” The gatekeepers of research resources, scientific journals, and the like seem to share your perspective: that any approach that deviates from current scientific orthodoxy is on those grounds de facto unscientific and therefore barred from access.
Thus I think the OP has a point - it seems an odd critique that ID research does not appear in those peer-reviewed journals which have a firm commitment to reject at the outset any research that supports ID.