We have similar bars at Treasury preventing grant recipients serving on advisory committees at the same time.  The thinking is that if the agency is paying you in part or in whole plus you are advising it, GAO and other auditors don't see you as independent but as a de facto contractor employee.  From the audit perspective, it can present an appearance of cronyism and unclean hands diverting dollars from a treasury account.

EPA would be best served by bringing talent in-house under one of the many "critical pay" authorities.  It would stand up to internal audit a bit better.  Temporary appointments under "critical pay" mean that up to 600 people across the federal government can be brought in and have their industry salary matched.  Intergovernmental Personnel Act, Space Act, and other authorities would make this pretty simple too.  Leave of Absence at the normal job while the feds pay you directly for the project duration.

Yes, I'm pulling a James Webb move here.  When he was SecNav he had a couple JAG officers comb the statute books for forgotten enactments empowering him as SecNav.  Don't look at this as a setback but rather grab somebody at the law school to help you round up forgotten programs and authorities to keep going.  If you need a librarian and can pay me I would have to resign my post as I would be switching sides even if it were for The Greater Good Of Humanity.

Or, you could run for Congress.  it would set your research back 2019-2020 but doing a single term could be fun.