You don't have to keep the trigger down to keep the checkpoint delayed. The thing which is delaying the checkpoint is the presence of active ordnance (see my checkpoint delaying page). Thus you can fire, wait as it flies through the air, then fire again. I often use a fuel rod shot because it'll spend longer in flight, so I can wait longer before the next shot is needed.

Of course, you can just keep the plasma trigger down if you want (and in confined spaces that's the practical thing to do), but it'll be way noisier, and if you've got vibration feedback turned on, your controller's going to be doing a lot of shaking.