Electrically heated blankets exist for various industrial purposes: keeping concrete warm while curing in winter; thawing frozen soil for winter interment in a cemetery or for utility work; and yes, for melting snow.
Powerblanket is an example. No affiliation, but I do own one of their small blankets. I've used it for melting snow off a 3x4 foot area of asphalt shingle roof and softening the shingles to make them workable so I could install a vent pipe flashing. I've also used it for thawing frozen soil. It works great, but buying enough to cover 200 sqft for a one-off roof job could be prohibitively expensive. An equipment rental center near you may have something similar available.
You could put a BIG heater in the attic, or leave the access hatch open so that heat from inside the house rises into the attic. But both of these run the risk of creating an ice dam on the roof that will be even more problematic and damaging. Note that it takes 144 Btu (or 42 W) to melt one pound of ice (or snow, I presume). If we use rough figures and suppose one cubic foot of your snow weighs 20 pounds, then your 200 sq ft of 1.5 foot deep snow weights 6000 pounds and will need about 864,000 Btu (252 kW) to melt it. A single electric space heater in the vented attic isn't going to do anything measurable.
Fifty feet above ground is quite high -- how will the roofer access and perform his work? It might make sense for you and the roofer to share the cost of renting a boom lift for a week or two. You could use it to access the roof with your snow rake and then wait for the sun to do its thing, apply external heat, or cover with ordinary concrete curing blankets outside and heat the attic to finish the job. The roofer could then use the lift to assist in his duties. (photo: jlg.com)
