Report on the Tea Infuser for Ground Coffee idea:
It works, with certain compromises. Purchased a stainless steel mesh "ball" tea infuser at Walmart ($2.50). Can only fill half the infuser, which appears to be around 3/4 oz (in volume) of ground coffee, if you pack it in. I'm accustomed to 1 oz (volume) ground coffee per cup. I tried the infuser with 1 cup of water. Let it steep 3 minutes in hot water. The coffee was good, not bitter as expected, only a bit weak, so next time I'd only boil 3/4 cup water (OR work harder at filling the infuser fuller). Tastewise I think it is closed to a french press coffee. The ball's diameter may present a problem with sufficient depth in the cup for a good infusion, so a narrow cup is best.
I ground the coffee maybe too fine for an infuser, so the last swallow had "sand" in it.
It could work with a few tweaks to technique, I think. A lightweight, low-cost, reusable solution.
Since the tea-ball opens into two half spheres, a secondary use would be to place the infuser over the neck of your water bottle and use it as a strainer.