I finally had the courage to rip up the floorboards in Alexander's bedroom to begin the task of wiring the baseboard heater:
Two floorboards have been removed from the closet wall (top of the photo) to under the window (bottom of photo), exposing the hundred or so year old hot and cold supply pipes.
Removal of the two iron pipes reveals the shallow notches cut into the joists, as well as the lath and plaster of the living room ceiling below.
Short sections of two by six (extending about six inches left and two inches right of the floor opening) have been sistered up to the existing joist using PL-Premium and a few 3 inch screws. These provide virtually no additional strength, but will provide the openings through which electrical cable serving this room will be drawn.
The first run of cable, a 12/3 NMD-90, to supply the 240v baseboard heater. Additional cable serving outlets and ceiling pots in the living room below will also run along this opening.