Thanks for the further details. Shooting off the shoulder while imbedded is a completely different application and now I understand your desire for the X50.
Invisio makes an optional iPhone adapter, so that might be one to explore.
Have you looked at SkyEye Comms on eBay? They make a variety of custom cables for the X50. The problem you are going to run into is that the Invisio cable has a chip in it to detect the radio, and this is why I feel that aftermarket cables may not work properly.
3M Peltor makes in-ear tactical earplugs that amplify ambient sound, and the latest version TEP-200 can accept 3.5mm audio inputs through a neckloop adapter. Not sure how you would do two inputs, but a y-cable, with one wire to a listen-only jack on a shoulder mic, and the other to a Comtek, may work.
My experience with in-ear tactical earplugs that amplify sound is that they do provide good situational awareness, but they do stick out a bit. (Probably about the same as the Invisio earpieces.) You would lose the bone-conducting microphone ability of the Invisio X50 system though.
My other live-fire trainer loves her in-ear tactical earplugs, but I hate them. I stick to my MSA Sordin earmuffs, and can run an audio cable to the 3.5mm jack directly into a mic with a listen-only jack, although in most cases, I just wear the acoustic tube headset underneath them. I get strange looks, especially when people don’t understand I can amplify ambient sound and hear them talking about me from a block away!