Wow, that is an interesting thought until you brake it down. Although I really like your way of thinking it won't really work that way.
First you need to know some definitions
1) Lumens : measure of perceived
2) Radiant flux : measure of total emitted light
As far as getting continuous energy goes that isn't going to happen. If you did bounce light back and forth of of mirrors once it hit the solar panel the light energy is then collected and it wouldn't bounce around anymore. Ending the perpetual cycle. And don't forget your using electricity constantly to make the light.
And for multiplication purposes; as light travels its intensity is lost the farther it travels away from the source. The more distance that is put in between the source and the receiver (solar panel, or plant) the less intense the light will be. So while putting up mylar or mirrors does reflect the light adding to the lumens, 100% of the light isn't reflected back. Some light energy is absorbed and lost to heat, some will reflect or refract in another direction.
The true use of mirrors and light:
To get the most out of your light, you want to contain the emitting light and all the radiated lumens in one direction. Any light that escapes the initial reflector in any other direction (like a wall or the floor) is basically lost energy. By putting up mirrors or mylar you reflect that light (the lost energy) in the direction where it can be used (plant, solar panel). This doesn't really multiply your light it more like adds. But the relfected light will be no where near as intense as it was when it was originaly emmitted. It will also be less intense than the light that just goes from light bulb to collector, rather than light bulb, reflector then collector.
Basically the farther light travels the more energy it looses. (distance not displacement)
