does anyone have any advice or practical experience of designing a gas lifted oil producer where the lift gas is injected down the tubing and poduction is via the annulus.

yes-  and it's usually a retrofit gas lift completion.

one way is to run a small diameter tubing (often Coil) inside the existing tubing( a bit like a velocity string)  and pump gas down the smaller tubing.  For the design process, it's the same as conventional gas lift design, except you have no unloading valves, so your deepest lift point is governed by your injection pressure and the fluid in the well before you start injecting lift gas.  BJ ran this type of completion sucessfully for me in a couple of wells, (using old coiled tubing, so the whole thing was pretty economical!).

I've also worked with other more complicated retrofit gas lift wells, where the gas was injected down a retrofitted tubing string set inside the old tubing and the oil produced up the annulus. As the old production tubing was quite large (5-1/2") there was space for unloading valves etc in the new 2-7/8" insert string.  The gas lift design was exactly the same as a normal design (most of the design software like Propser can do annulus and tubing flow at the click of a mouse.  

The trickiest part of the project is ususally the new wellhead spools and tubing hangers; as the lead time from Cooper Cameron or Vetco or whoever for these unusual bits of kit can be quite long.