Another option would be a crankbros sage 1.5 directset (bearing and cup are one part)There is one really sh!tty FSA headset that will allow you to run a 1.5 steerer in a 49mm headtube - http://www.universalcycles.com/shopping/product_details.php?id=36972&category=726
You would have to ditch the upper cup and use a standard 1 1/8", but it is cheap anyhow.
Most traditional external headsets have approx 15-16mm of lower stack
Most "zero-stack" headsets acutally have 4-6mm of stack (including that FSA one)
The crankbros sage has 10mm of lower stack
So best case you'll get 10-12mm (1/2 inch or less) lower front end which is approx 1/2 degree of headangle.
With the crankbros setup, you'll get half that = 5-6mm lower front end ie. not much. however, i doubt it's possible to run a 1.5 with a "zero-stack" on this frame so i'd bet this is the lowest 1.5 setup.