Curious myself as this sounds like a great hike! If I remember correctly, the Magalieberg section of the MCSA did a day hike up to the highest point in the North West, not too far from the towers (which fall in Gauteng), but I don't think they did it from Upper Tonquani approach. Then again, I remember hearing something about some confrontation between a farmer and hikers trying to get to the towers, but I think that was approaching from Breedt's Nek.
I would take this up with the Joburg section of the MCSA since they control permits for Tonquani. I think Roy Kendall is the land access guru, but I'm sure Uschi will point you to the right direction quite quickly.
Not sure about the common access rights to the summit, especially the closer you get to Hartbeespoort and Pretoria. In practice those sound like tricky areas. I think access from Bergheim kind-of works this way...my impression anyway.