Hi, rjk. It's a great pity that this interesting project was finally failed.
You should have made more progress if you had asked help from AMD directly.
I heard there was an informal "Multi-GPU" project proposed by AMD last year and finally
they got 16 GPUs (8x FirePro S10000s) to work in a single rig successfully.
So I think AMD might have already developed a proprietary driver to overcome the 8-gpu limit.
See:
http://devgurus.amd.com/thread/158863
http://fireuser.com/blog/8_amd_firepro_s10000s_16_gpus_achieve_8_tflops_real_world_double_precision_/