I'm guessing at this one, so bear with me......just going off of experience is all:
Pilot reported Low HYD PRS prior to landing. Right side Hyd Sys was empty of fluid. Still no cause for that but obviously it leaked out during the flight.
So, the left side should have compensated.
If I remember correctly, it was Continental that did a wheels up landing in Houston about ten years ago. One of the outcomes of this was that all DC-9 and MD-80 acft with the HYD "HI, Low, Off" switch, would ensure that the manuals were reflective of this since flaps and landing gear would not work effectively if not set to high.
This made me think for a minute........then:
If the right side was kaput, and the left side engine had been shut down for taxi (DC-9's have an engine driven Hyd Pump), and the switch was off, then they would've had no brakes for the taxi.
Statements by witnesses say that the captain had used his thrust reversers just prior to impact at 15 mph. This tells me no brakes.
Anyway....I'm assuming for now that it was as I mention.....we'll just have to see.
Any word at your end Jet?