Again, I'm not expecting anyone to know the answer to this question anymore, but:

Is there a way to use Deployments/Loadouts outside scene editor and buy menu? As in is there a .lua function to, say - spawn a certain Loadout (when a faction is provided)?

Hmm, I've never seen any work in this area and I'm not sure if there's honestly anything you can do with this stuff.

Looking through the wiki I'd say look in PresetManager or maybe BuyMenuGUI. SceneEditorGUI could be useful too perhaps, though I doubt it.

Alternatively, you could try digging around some base files and seeing if there's anything helpful, or pming someone involved in it (weegee would be a good bet I think).