I have kept multiple cats in the same house with snakes for 15 years with only a few problems that were all entirely my mis-management. The last 3 yrs I'm had 6' boas with cats of varying ages, including 2-3 day old kittens.
First with what you are saying you are going to do: keep the unattended kitten in a carrier and lock the boa cage in a closed door room you basically would have no chance of injury to either.
If I read you correctly you are asking if a boa possibly a threat to a kitten, or vice versa. Yes, both ways.
A kitten will grow beyound the size of a meal before the boa is large enough to eat it. Eventually, the boa (depending on species) will be large enough to eat a cat. Now, will that actually happen? Probably not. I think a snake if in it's right mind only kills to provide a meal for itself. Could a 7 foot long boa constrict to the point of killing an adult cat? Absolutely. But, is it likely to happen? Not if you don't ever allow the situation to occur that would precipatate that event.
I've had my cats kill baby corns and trantulas. Entirely my fault on both occasions due to MY choice of suitable enclosures.
Cats will sit on top of the enclosure at times so make sure it isn't screen-it will break.
Use safe, common sense practices and you shouldn't have any trouble.
Please don't keep a kitten in a carrier for "the time you are away". dogs are denning creatures, but cats are not.