I tried to make this happen.
Since even Charon cannot mod it, I tried to give some kind of "workaround" trying to make inventory management (especially regarding shields and basic ammo) easier without breaking game balance.
Tested settings that really affects these; NG+2 except: alien weapon damage 1.05 (1.0 on NG+2, 1.10 on -) @ gameconfig.xml
Basically your dropship have different "lockers" that can be accessed entering dropship (improvise on cases there is no physical dropship available, like on Alien base "dropship" is starting/exit area, Xenonaut base that is Command room...), imagine that dropship pilot (or something) supplies these items. Accessing any locker (except med bay) ends soldiers turn however (AP=0, mod it if don't remember soldier accessed locker) but accessing locker does not take action points (ie. all lockers can be accessed on single turn). If using locker requires more action points that soldier has (to switch items from backpack into hand for example), modding more action points is allowed. Switching equipment between soldiers (any soldier that has equipment can access locker) is allowed as lockers do not provide more items into battlefield. Also, if there is no aliens on sight for 5 turns AND soldier has "safe route" to craft, locker can be accessed without actually walking into craft. Basically locker is not available when there is active combat going on.
- Shield locker: To access shield locker, soldier must have had shield on left hand when battle started AND have shield on left hand when accessing locker. Shield locker provides unlimited amount of undamaged shields and even replaces totally destroyed ones (does not increase amount of shields on battlefield however). Naturally this does not apply to alien shields.
- Med kit locker: To access Med kit locker, soldier must have had some kind of medikit or first aid kit when battle started AND have that kit on inventory when accessing locker. As shield locker, provides unlimited amount of medical supplies for soldiers that enter craft.
- Ammo locker: To access Ammo locker, soldier must have had enough clips when battle started for weapon that receives unlimited reloads from locker AND have that weapon on inventory. Multiple weapons allowed. Applies to weapons that have unlimited clips available on base (NOT rockets, grenades, Alien weapons etc). Clip requirements are: 5 clips if clip takes one space (like pistol clip), 4 clips if clip takes two spaces (like Laser sniper clip), 3 clips if clip takes 4 spaces (like Laser battery for heavy laser weapons). Clip(s) may be dropped after battle has started, locker is still available. Clip requirement is only to "waste" inventory space from something more important, ie. balancing.
- Med bay: Any soldier that enters craft, receives free med kit that has unlimited healing power. Does not end turn. That med kit cannot be carried out of craft however and using it still consumes action points.
How these actually work? Start game using Xenonauts_gc_editor.exe select soldier that wishes to use locker and launch editor during battle pressing U-key. Notice that editor will show alien locations, you can hide map window using Task manager always on top or something similar (map is not really needed). You can see what soldier has on Left Hand or Right Hand. Press Reload and ammo/shield/med kit/whatever charges are restored. If shield is totally destroyed or soldier wishes to access Med bay, select appropriate item from drop down menu.
About balance. Even if soldiers have "unlimited" shields, they only get "unlimited" shields outside active battle. Also aliens hit so hard that soldiers are far from invulnerable. This is practically solution to "carry extra shields, drop them when battle starts and run to get them" -hassle. Ammo requirements are only to keep balance, so that soldier cannot take heavy weapon AND tons of rockets+other stuff. So essentially heavy weapon means much less other things even that weapon has unlimited ammo. Also it's not bad idea to keep spare clips on middle of battle, locker is for "quiet" situations only. For medicals, carrying medical supplies is advised, "craft" may be pretty far and single kit have quite limited healing power.
Overall I cannot see this breaking balance any way. While basic items are "unlimited", special stuff is not. This makes game much more enjoyable though IMO. Feel free to improvise.