It feels a little restrictive to always have to equip champions via the store, and it's quite frustration to have 10+ horses stored but your champions are stuck on foot due to lack of gold. So here's my solution.
Create a new unit type (like how henchmen are a different unit type). Call this type "Equipment". In the unit design screen, you select armor, weapons, and support items (like mounts or rings), but cannot select Traits. Once you save an Equipment-type unit, it's inside the trainable units list, but is greyed out unless there's one or more champions in the city.
Once you choose to train it, you are prompted to select one of the champions in the city (like how you select the recipient of a strategic buff spell). This champion is immobilized until the "unit" is completed, then they receive all the equipment built, and are free to go.
Maybe it's a little inelegant, but I can't see any superior alternatives, (though leaving it alone seems to be the current direction). Thoughts?