Ok, my biggest problem is setting the sysvar's, it seems you need to do some gymnastics to set them, this is what I have so far but it's still failing, I'll have a bit of a dig around in the meantime.
'set the clipping planes on:
Dim distF As Double, distB As Double, vmode As Integer, vardata As Variant
distF = 500#: distB = -500#: vmode = 6
vardata = distF
ThisDrawing.SetVariable "FRONTZ", vardata
vardata = distB
ThisDrawing.SetVariable "BACKZ", vardata
vardata = vmode
ThisDrawing.SetVariable "VIEWMODE", vardata