I think you guys are making this out to be much more complicated than it needs to be. If you are just trying to listen, that is pretty easy with an IFB. If you are looking for a communication solution and need to also talk, it is a lot more complex.
I think your best bet is to use the packs provided. Otherwise you need to purchase your own wireless comms system (wireless pack and base station), then integrate it at every location (assuming you are working multiple locations). I recommend you look at HME packs. I like the BP850 pack the most. It offers multiple channels. You would be able to configure the primary buttons to talk with truck - You can also assign a secondary button to speak pack to pack (you would be able to put your assist on a pack and have private conversation). You can get a belt pack with either 5 or 4 pin. Personally I would purchase a 5 pin headset and carry a 5 to 4 pigtail for when needed.
Comms companies don't play along, just like camera companies don't play along. You can't throw a DVCProHD tape in an Arri Alexa, and they will never develop a single form of media...
Even with the pinouts correct, you will still run into issues depending on what type of mic is in the headset (dynamic, electrolet, etc...) On a camera you can adjust that by a menu, but with a belt pack, you often need to open it up and change an internal setting - if you can change it at all.