I think it's against the law to void a warranty for a part of a product that's unrelated to your modifications, but check with a lawyer to be sure. AFAIK, if they want to void your warranty for modifications, they have to link your modifications to a warrantable failure. I.E. if your blade snaps in half, they would still have to warranty that unless somehow you damaged it when tapping your thumbstuds, or the act of tapping your thumbstuds weakened the blade to the point it broke easier (they would be able to deny it in this case). If say for instance you tapped the thumbstud and your knife handle broke, they should have to warranty that (assuming there was a warranty covering that in the first place). So it would likely depend on the failure that you want to get warranted and whether your work affected or influenced that failure.
Ask a lawyer about the Magnuson-Moss act, or google it.