@Austin Excellent! SFF got back to me and said I could apply with pending 501c3 status, but since the form still asks for an org with documentation proving 501c3 status I'm putting down Manifold for Charity and linking to the IRS determination letter, with a note about Luthien's pending status.
Would it be okay if, in the event SFF wants to issue a grant and Luthien's 501c3 status is still pending, I reach out then to confirm the fiscal sponsorship details? (If this happens, I'm totally fine with the 5% overhead.)