There are two issues: overall length of the negative contact, and the pressure required for proper operation of the QTC.
Peak's momentary switch consists of the following parts:
(1) Switch body with an external o-ring. These bodies have been manufactured only in SS or brass, and in just one size. The body is threaded either directly into the body of the light, or into an adapter, depending on the flashlight body.
(2) Button with an o-ring. SS or brass. Just one size. This is inserted into the aft end of the switch body. The front end of the button has a female thread to accept the battery contact piston.
(3) Button return spring. This beefy spring is housed within the body of the momentary switch. When manually compressed, it allows completion of the electrical circuit. Spring tension at all other times prevents battery contact with the negative terminal.
(4) Piston. Made of brass. At its aft end is a male thread. The piston is inserted into the front end of the switch body, through the button return spring, and then screwed into the front end of the button. This locks these four components together. Pistons are made in three different lengths depending on application. The front end of the piston is bored out to accept the next component.
(5) Piston spring. Tension from the spring itself keeps it inside the piston.
(6) Battery contact button. Brass. Inserted into the piston spring and also held in place by spring tension. When the operating button is depressed, the battery contact button moves forward to contact the negative terminal of the battery, and the small piston spring exerts enough pressure to maintain the closed circuit but not so much that there is any chance of damage to the battery.
These last two components, the piston spring and the battery contact button, have been eliminated in the momentary switches for QTC lights. The reason was simple: the presence of the spring reduces the pressure available for activation of the QTC. In QTC lights, the piston is solid and sized specifically for the travel the QTC requires. A pocket clip which is mounted forward of the momentary switch places the switch further aft and therefore requires a longer piston in order to reach the QTC.
Peak has produced threaded pistons over the years in a total of four different lengths and its possible one of the older, longer pistons would work fine with a pocket clip. Some time ago in one of his clever Peak mods, MWClint simply replaced a piston with a properly sized rod made from a brass nail.