First off, any bullpup stock that allows the action to fire separate from the stock is a prohibited device. That renders all the cool SKS bullpup conversions, the 10/22 stock you posted and all other aftermarket bullpup stocks as prohibited devices.
The definitions you seek are not in the FA, they are in the criminal code under Part III - Firearms and other weapons:
http://www.canlii.org/en/ca/laws/stat/rsc-1985-c-c-46/latest/rsc-1985-c-c-46.html#PART_III_FIREARMS_AND_OTHER_WEAPONS_305898
Read the definitions at the beginning of the section for the rules on prohib and restricted firearms.
There are also additional regulations that are referenced from the criminal code, these are where rapid fire triggers, bullpup stocks and others are listed and mag capacity limits are specified.
http://www.canlii.org/en/ca/laws/regu/sor-98-462/latest/sor-98-462.html
Way near the bottom of the page is the bullpup clause:
2. Any rifle, shotgun or carbine stock of the type known as the “bull-pup” design, being a stock that, when combined with a firearm, reduces the overall length of the firearm such that a substantial part of the reloading action or the magazine-well is located behind the trigger of the firearm when it is held in the normal firing position.
I believe the "when combined with a firearm" statement is what makes aftermarket stocks illegal while bullpup designs such as the Tavor are legal. The Tavor (and RFB) does not have a separate stock as such, the receiver forms the stock and the rifle cannot be fired when disassembled to the point where the "stock" components are separate from the rest of the rifle.
