[Andy A.]

To answer your question, No, there is no rule against what you did. You can modify the joysticks in most anyway you please.

In fact, the rules regarding your control system are pretty sparse. There are some limits on what kind of power you can draw from the OI, regarding the use of circuitry to process your commands, and of course a size restriction. A few others, but nothing regarding the joysticks in the kit.

My team, and I think the majority, have used Velcro in the past. It helps facilitate switching joysticks around, since they are cheap and tend to break. But I've seen teams that mount them like you have and done fine with it.

-Andy A.