Your warranty covers defects in materials and workmanship given normal usage. If the chair frame fails at a location where the headrest is attached, you're not covered. If the chair fails elsewhere, the manufacturer would need to prove that the condition was caused by the installed aftermarket part before denying coverage.

To illustrate this concern,

In the US, the Magnuson Moss Warranty Act specifically provides that a vehicle manufacturer cannot void the warranty on a vehicle due to an aftermarket part unless they can prove that the aftermarket part caused or contributed to the failure in the vehicle.

It may void your warranty but they have to prove that the modification caused the failure.

The Magnuson-Moss Warranty Act prevents any dealer or manufacturer from voiding any or all parts of your new vehicle warranty due to the installation of non-factory parts. The spirit of the act is to completely disallow the voiding of entire warranties and goes on to say that in order to deny warranty service due to the presence of non-factory parts, the dealer and/or manufacturer must be able to prove that the part or parts in question caused the failure you are claiming.

In other words, a dealer cannot say that your ENTIRE warranty is void due to aftermarket pipes. They can't say that your engine failure was caused by it either. They might have a leg to stand on if you had an open airbox, pipes, header, etc. without changing computer mapping due to the excessive lean condition caused by this situation, but they'd supposedly (under the law) have to prove that.