Biggest danger seems to be falling. This idea is simply to have a portable adjustable safety cage to prevent this in case of emergency. Think a sort of rally car roll bar meets a kids football net. Made of high tensile hollow steel with stell mesh - should provide strength with portability.
Using the tubular concept the length could be adjustable for 1 - 3 people depending on weight. A steel cable would be on each end and secured to the opposite corner of the roof via clamps.   On sloped roofs it would function to catch the worker if in danger. On flat roofs more to shield the worker  from wind and the like.  Mesh concept prevents the paracute effect of blowing away in high wind.  Could also be able to have a removable plastic floor to hold tools and other equipment.  The pictures are a bit crude ( basic photoshop hack together) but show the basic concept. Obvioulsy on the sloped roof version there would be more catchment area if a fall occured and the design would be better.  You could also have the exisiting safety anchor weights system able to attach to the cage and so a second one could be placed in the middle of the roof to act as a replacement anchor point when not in use.  

Not sure if the blue cage like picture is a similar concept already in use - if so please ignore the idea !!     :.)