Creating living gear for our people isn't beyond the bounds of reason, but making sentient creatures intended as spellcasting fashion accessories is crazy, and not in a good way. It would be less ethically dubious, and more useful besides, to simply grow servitor creatures, such as Advanced Cleric Creature Lotus Leshy, to assign to the Legion.
I don't really see it as ethically dubious, the spell casting snake, is just using the Legionnaire as a battle-mount, while he's using it as a supporting caster, it's not like it's going to be his belt 24/7, riding around on him and casting spells for him, is just its job, when it's not on the job it can do whatever.
And yes servitor creatures of a higher level are more useful, but they're also far more costly, a 1-2HD caster snake, only cost us 3-6HD worth of corpses, whereas creatures that can independently support the Legion cost far more, and most of them have to be made with reagents, as we can't get ahold of high enough CR corpses.
The belt snakes, are meant to be an affordable way, for all our Legionnaires to have some support, although maybe belt snakes are an inappropriate word for them, they aren't really belts, they're simply snakes that coil around a Legionnaire, so they can assist him in battle.
Beings like Advanced Cleric Creature Lotus Leshy are awesome, but they're also expensive, and so not really something we can make sure every small squad of Legionnaire has with them, the belt snakes, are meant to be cheap, so ideally every Legionnaire can work with one.