...I'm stuck away from my modding/gaming rig until tomorrow, and want to continue theorizing/planning my next modding project and need to know something before I go and waste my time. In the GECK, will an armor still show up on the playe if it has no biped slot selected?

I gave this a try, making slotless recon armor and a pair of reading glasses, but neither would show up in game, unfortunately.

The editor will warn you when you have a wearable item with no slot selected when you load the esp. I have used items like this many times to run the equip block without actually equipping the item, and since it doesn't use a slot no items are unequipped in the process.

I'd be willing to give up an earing slot for shoulder missiles though.

Problem is, a lot of different mods use this method, and so compatibility starts to become a bit of a headache.

If you do not assign a slot, there is no compatibility issue.

In order for it to show up on the character, there must be a slot assigned, otherwise the game does not know where it goes.