I often end up having 2 to 6 different 'outfits'.

Here are some I use most often(even more, depending on class and items found):

1. Melee fighting
2. Missile combat
3. Travelling(in dungeons)
4. Spellcasting

5. Even one for travelling in the wilderness, but not used as much as the above.
6.

I was wondering whether there could be a shortcut added to quickly change between these different 'outfits', rather than having to find each piece of equipment in inventory every time.

Obviously the usual turns will pass while re-equipping.

Cursed equipment will also have to be taken into consideration. Also order of original equipping, for example girdle over or under body armour. The easiest way would probably be some sort of macro that link to the items in inventory.

An example of shortcut keys could be; the usual 'i' followed by '0-9' for the selected 'outfit'. Re-equipping only takes effect upon pressing 'z'. So, if the player decides he would rather keep the current equipment equipped, (s)he can just press corresponding number key and 'z'.

I hope everyone understands what I'm asking for, if uncertain, I'll try to use different words.

Footnote - I would certainly play a lot more if this is implemented, currently I have to take very long breaks as even equipping just a weapon on each encounter is too much to handle for long.