Can I run a T5HO ballast with significantly lower wattage T5NO bulbs?
That's not how it works.

Unlike incandescent bulbs, florescent bulbs do not draw a particular amount of power - they are rated to handle a particular amount of power. They are actually "negative resistance" devices - which basically means they will draw an infinite amount of power if you let them. (Specifically: The more current that goes into them the lower their resistance, which then allows even more current to flow into them, reducing the resistance even more.)

The job of the ballast is to prevent them from doing so and refuse to feed them more power than they are rated for.

So it's the ballast the controls the power NOT the bulb, and if you tried what you are suggesting you will put 54W of power in a bulb rated to handle 13.