It's possible, the main draw-back is the loss of the already limited Z travel when doing normal work - this is a big pain in the wotsit really. Not so bad on the aux spindle as all the tools are so short as is the work itself.

To make it really work would need the knee setting up as an axis and diverting all tool-length measurements to that axis - this would mean cutting is done on the quill but the loss of travel would not be as serious.

I still feel the quill rack is a way forwards, won't cost much to try at least.