To fasten things to metal studs you need to use screws and you need an electric/battery powered screwgun/drill with a good screw bit in it . Any thing is doable if you have the right tools

Well, I will have this conversation with the wife, but I have a feeling I know how it is going to go

Steel studs aren't a major obstacle. If you only modify one stud bat and stay a bay or two away from the ends of the wall I wouldn't worry much. Just screw in any shelves and rig a piece of plywood, skim coated with drywall mud and painted so it blends, to make a door.

If there is any structural issue or the wall feels shaky it is simple enough to either buy similar steel studs and to make a square section or insert wood ripped down to make a filler.

Lots of room in those empty stud bays. Even more if you fur them out to make a 6" bay. You can also make other improvements. Furred out a bit you can insert isocyanurate sheets as thermal insulation. cover the inside of the bay with ice shield sheeting and it can increase the acoustic insulation. No need to hear your mother in-law having a blow-out.

Other areas that can be converted to storage are soffets in the kitchens and bathrooms. Gasket any doors to keep moisture and fumes out. Doors are sold at many home centers. Perhaps add a little insulation. Use some discretion. Some soffets are sturdily built while others are capable of little more than carrying their own weight. There is also an issue of making sure there is a barrier between the soffet storage and the attic.

Another spot is hallways and closets. Both are typically the same height as the rooms. In many modern houses this is several feet taller than what is normally used.

A 10' hight living room leads to a 10' hallway. There is no reason the hall needs to be 10' high. Build a frame and drywall or plaster to match and you could have a storage area overhead the length of the hall.

Access can be by way of overhead hatches or by way of doors cut through the walls of adjoining rooms. This later method is usually best for using the unused space at the top of closets. Either way you need a step-stool to get to your stuff but there is a lot of room.

Another option along similar lines is to install cabinets along the junction of the wall to the ceiling. This space is usually unused. Well done it looks good. Lots of space for supplies or goods. Particularly the light and bulky stuff like sleeping bags, winter clothing, paper towels and, the one thing your going to need long-term, toilet paper.

Can't believe it!
I threw this out to the DW and she actually liked it!
As long as we're not storing the pork-n-beans in the bedroom, I think there will be "in the wall" storage in the future.
Thanks for the idea.

Sneaky! Sneaky! But I like it! You've got many Good Sound Points there!

(Also reminds me of my Kid Days watching Batman, -how when they turned a Bust or Statuary Head, -a Wall would turn around in the Wayne Mansion, -to enable their entry into the Batcave!).