Yeah if you have a good solid culture going you could definitely use that. If the culture is just LABS there shouldn’t be much gas produced(if it all), but it is going to be rare that it’s completely clean.
I’d say with that method if you don’t want to have to touch it you could easily put an air lock on it. That will allow gas to escape without allowing anything to get in. Inexpensive and widely available from homebrew shops. They make some that fit mason jar lids as well.
If you find a need, you can also procure lactose from a homebrew supply. I’ve never done it, but I don’t see why you couldn’t culture with that if you want to. I don’t think you need to do that if you have a solid culture to work off of already, but if you ever lose that culture and you want to get back to LABS dominant without using any kind of dairy that could be a good option. Likely you would want some kind of addition of a mineral source or something to keep the culture really healthy(unsure on this as I have had the need so I haven’t gotten that far into the minutiae of what lactobacilli need for optimum health). I just figure anytime you use something so isolated, you usually need something else to round it out for optimum health. I small bit of grain or molasses or something. The beautiful part of the lactose is that other than lactobacilli, there aren’t too many things that can digest it.
Fun milk fact, raw milk contains enzymes and cultures that help break down the lactose and make it digestible. Most people who are lactose intolerant will have no issues with raw milk or dairy products that have not been pasteurized.
Sorry for being so long winded. Hopefully some of the info is useful