OK, LARGE contractors can get this done, with additional wording that the endorsement is limited to the operations of the insured (that would be YOU). So you might be able to talk a carrier into it, with that additional, limiting, wording.

In order to understand why carriers don't want to do this, let's look and what each section does:

Hired auto: this extends YOUR liability to vehicles you rent. Depending on the symbol, it might be limited to private passenger autos only, or it might be all inclusive. If you add the gc as an additional insured, without the limitation above, you're ALSO covering any autos HE rents. Technically, that COULD include heavy equipment licensed for road use, that he subcontracts from ANOTHER subcontractor. So once you add the gc, everyone else gets tagged along with it - MONGO exposure, that's really hard to assess, and almost impossible to get the premium for, as the GC's cost of hire is NOT included in the rating factors.

Non-owned auto - this extends your liability to BORROWED (as in, you're not paying a fee for rental) vehicles, including (but not limited to) employee vehicles. Again, it might be limited to private passenger type autos; it might not. If you add the GC as additional insured, it means your policy covers HIS borrowed vehicles, including HIS employees vehicles, and worse, his other SUB'S vehicles. Another impossible, unratable exposure.

What I would do if I were you . . . 1. see if the carrier will add it on, with the limitation wording on your cert, "limited to the operations of the insured". That will exclude the GC's ops, and his other sub's ops. 2. Go to your auto carrier, see if you can buy a hired & nonowned auto policy ONLY, that ONLY covers PP type vehicles, with that endorsement - it should run you about $250, and another $50 for the AI endorsement. Then add the $300 extra into the job cost. Assuming you've talked it over with the GC's RM, and that will work with them.