It is another form the ていい/てもいい grammar point we talked about before, so viewing it as the だ becoming conjunctive is likely accurate.
Edit: actually, on reflection, it’s the で particle, which has a lot of uses. It doesn’t make sense for it to be conjunctive here. I apologize.
Edit the second: No. Nevermind. I should have trusted my gut. I lean to it being conjunctive. (Sorry, I’m indecisive because で is one that throws me as well)
Somebody else asked about this at some point too, and this was my answer to them (click the arrow for the spoilers to show up. Preview just hides them):