This seems a realy big problem and a severe pain in the rear!
Can't believe that non of the usual techniques to bleeding a system work at all.
As I've said before to you H, you can always try the drainage valve at the bottom of the rad providing its there which I'm sure they all have although could just be TT
There's been times with severe airlocks on my mates system and we tried the valve to let a lil air out, next minute we had a eruption out of the top of the rad. Seemed to be ok after bleeding and pressurising the system with his rad kit thing, cept water pump was naffed so changed that and all was well. Not sayin your pumped buggered but I could ask whether I could borrow it from him if you wanted to try it?