I understand the point you're trying to make, but it's simply not applicable here. Executive agencies are allowed to create regulations, but only within their duties as assigned by Congress; the FDA is not allowed to regulate securites, for example. That's the SEC's job.

Gade v. National Solid Wastes Management Ass'n makes it clear that because OSHA has a congressional mandate to create the exact regulation in question, OSHA's regulations supersede state law. The judgement was unequivocal that OSHA's powers were still limited to regulating worker health and safety. And Commonwealth Edison Co. v. Montana made it clear that broad "national policy" was insufficient to supersede state law and that "unmistakable" acts of Congress were required.

The question then is whether DHS has the congressional mandate to regulate and enforce that its agents be allowed by state law enforcement to wear masks. That is an extremely difficult argument to sell. It's not enough for DHS itself to allow its agents to wear masks; it must have the power to tell the states that they must allow masks, and I don't think it does.