@Taxes-by-Rocky CCA's have taken the place of Revenue Rulings at the IRS opinion factory, and as they say "This advice may not be used or cited as precedent."  More importantly, this one says in two places that the answer "depends on the facts and circumstances of each specific case."  It does answer one question I had, about the origin of the "seven-day rule."  It relates to Treas. Reg. § 1.469-1T .  The "T" means that they expire after three years, unless issued before 1989.  Good luck figuring out the issuance date for that one.  But Section 469 deals with passive vs. active, not Schedule C vs. E.  

In addition to the two court cases I have mentioned above, the CCA refers to  Johnson v. Commissioner, 60 T.C. 829, which involves "boat stall" rentals; and Hopper v Commissioner, 94 T.C. 542, which involves storage units.  The two fact situations in the CCA are not the same as the one described in this post.  Nevertheless, it's a good place to start (back when I first researched this question, I think it's probably where I started).  But the message is, "we don't really know until someone disagrees with our position and takes us to court.  And even then, we might not acquiesce."

So if the activity is showing a profit, it's likely to be found on Schedule E.  And if it's showing a loss, it's likely to be found on Schedule C.  But practitioners should understand the risks and rewards of either position.