say your hanging a HPS 600w light vert, but its along a wall or in a corner. Say its surrounded 180 degrees by plant and 180 degrees by tent wall
Sure if you have a reflector already then use it. As long is there is enough clearance as to not burn/melt the tent by a hot reflector.
That said, I would NOT run out and make a special purchase for a hood for this purpose. The walls of the tent already reflect, so the only benefit from doing this is the fact that a reflector is closer to the bulb which means the reflected light is closer to the plants which means you're being more efficient with your wasted lumens. Not enough though to merit running out and buying a new reflector IMO.
Another option, one that I use as well, is tape sections of reflectix to get better reflection in the weird corners.