I can see part of where you are coming from in wanting to do this as a model plane taking off horizontally, that happens to use rocket power.
So if you were not doing this at a rocket launch that has to follow the applicable safety codes, you could perhaps call it a model plane that is rocket powered, if the model plane definitions allowed for it.
The real show stopper, for the U.S. anyway, would be the use of an HPR motor. HPR use is subject to rocket safety codes and rocket regulations. There is no exception for use of the motors for model aircraft. So, to use HPR motors, they have to be flown in accordance with the rocket rules and regulations.
Now, under the NAR R/C Rocket Boosted Glider rules, which are written in a manner to apply to Model Rockets and HPR, you COULD do horizontal flight if after an air drop it is ignited from a safe enough altitude (minimum 100 feet) to ignite the motor, and the model is over a safe area and pointed in a safe direction at the time of ignition (and a loud countdown given before the ignition).
I know, you are in Canada, so the govt. regulations are not the same. But you may have the same brick wall issue with the legality of using HPR motors for anything that is not used in accordance with HPR regulations.
Also, BTW, if you were able to find a way to use an HPR motor legally to take off horizontally as a model plane, then you also would need to address the issue that unless you flew this privately where insurance did not matter, you would have no insurance to cover you in case of an accident since model aircraft insurance would not cover it, and rocket insurance would be void.
So let's say you wanted to do this as a "model plane" taking off horizontally during a rocket launch. I think you're out of luck. Because the rocket launch organizer is bound to only allow use of HPR rocket motors in accordance with the regulations and codes that apply to HPR motor use.
So for U.S. flying, the limit for a R/C RBG (that can be controlled on boost) ground launch is 45 degrees for NAR, 70 degrees (20 from vertical) for TRA (they have no R/C RBG code that I know of). Horizontal for NAR only if done as an air start from a safe altitude, location, and direction, as per the R/C RBG Safety Code. AMA rules would never allow for HPR motor use.
Not unless you can work something out with the Model Aeronautics Association of Canada, and they would allow for the use of HPR powered "model plane" flights. But even MAAC cannot overrule government laws that would take precedence. I am not familiar with Canadian laws and regulations to know if there is more flexibility for that than there is in the U.S.
- George Gassaway