Unless I'm reading this incorrectly this is a peering arrangement, which is qualitatively different than just hosting a box in a DC run by Comcast (which doesn't exist, as far as I know, so I can't do it as a normal citizen). I could colo a box "near" Comcast, but I'll be more hops away (worse performance) and I'll have to purchase transit from some ISP that does peer with Comcast (more expensive).
Most (all) major ISP's do have DC"s that you can colo in (including comast). Just call up their business services line and ask.
So, yes, you can colo in one of your ISP's DC's. Netflix is doing just that (which is why it's not a peering agreement, it's a colo agreement). They just don't get charged because they are saving the ISP the money they would normally charge for the colo service (sometimes more).
Even if you can colo with Comcast (which I can't find any information about online) you're still not direct-peering with their AS, which is what Netflix is asking for with Open Connect. I'm sure they'll also charge you for bandwidth. This is definitely different than putting a box in a colo facility. This is putting a box in a peering location and eliminating the transit cost and performance hit.
