Cement resistors generate heat over a small area. So you would need to use either a very large number of them, or use aluminium heat spreaders to spread the heat from each resistor out over the zone you want it to heat. You would also need to use borosilicate glass instead of float glass to avoid the risk of cracking when you only heat part of it, and you would need independent temperature control for each heated zone in order to get an even temperature.
Not worth the extra cost and complexity IMO, except perhaps if you have a very large heated bed. If you want to save energy, I think a better option would be to enclose the printer to keep most of the heat in, thereby providing a heated chamber as well - which is needed when doing large ABS prints anyway.
Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod
Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].
