If Britain rides roughshod over NI by placing it in the EU customs zone without their consent there will be trouble, it would need democratic legitimacy via a referendum in Northern Ireland first. I doubt there are enough hard-brexit supporting unionist die hards to vote for keeping NI in the UK single market and customs union rather than the Irish one, so that consent will probably be forthcoming. NI would still be part of the UK though, just outside the rUK customs zone and single market. The DUP won’t like it, but the people of Northern Ireland as a whole will probably go for it given that moderate unionists will want it as well as almost everyone of a nationalist/republican pursuasion.
It really shouldn’t be up to them to pick and choose what sort of involvement any of the UK has with the EU, since the UK voted nationally to leave the EU.
Somewhat akin to our letting New Mexico decide that it wants to conduct business under Mexican trade rules, not those of the US.
If they really want out of the UK, NI, with Ireland’s agreement, can shift itself to Ireland. But of course, that’s not what they want.