Check out this web location: E-Lacrosse.com

Scroll down to the 3rd story (or tip) that's there. It's the tip regarding end-caps for sticks .....

Is that modified stick legal?

Thanks for checking that out

There is a lot of stuff in that page that I would not do if I wanted to guarantee a stick still being legal.

Now the tip about taping a bottle cap to the end of a stick:

The rule on but ends is a bit different than the rest of the stick. It is if you have on good, if you do not the stick needs to removed until fixed but no other penalty is assessed. This does not allow you to tape something dangerous to the but end of the stick. So if I saw someone with a bottle cap taped to the end of a stick and the metal edge was exposed I would probable call it as an illegal stick. Also at least in the NFHS rule book only wood, plastic, or tape could be used as a but end. This means that the stick has a illegal but end and the stick could be called illegal because the language about removing a stick is only about a missing but end.

In the end I would say do not use a bottle cap as part of a but end and if you do ask the referee before the game.

Bottle Cap Butt End
 

Also,

If you do this, as long as it's not dangerous to you or an opponent, it will be ok. If it is deemed dangerous by the referee doing the stick check (as in the example above about the metal ends being exposed) then the officials should just tell you to fix it (i.e. - take off the dangerous material and put on a new end cap or tape.) It's not a time serving penatly to have an illegal end cap or no end cap. You just have to fix the problem to the referees satisfaction before the stick reenters the game. If you fail to fix the problem, or refuse to do so, and the stick comes back into the game, then it will become a time serving foul.