I don't know about the Blackhawk loaders, but the couple OEM loaders I've looked at from Kubota, Yanmar, and Bolens came one of two ways. Either they used the hydraulics from a divider block at the three point valve, as you're trying to do (And tapping into the hard line upstream of the lift valve accomplishes the same thing) without any additional tank, or there is a standalone pump, driven off the crankshaft or via a belt, with a reservoir in one of the uprights.
This seems to break all of the guidelines for having the reservoir size equal to or larger than the pump's output in GPM, but every one of the vintage Japanese tractors I have or have looked at use one of those two setups. I've never seen an auxiliary tank in series with the transmission reservoir.