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.
It should be for all NI businesses outside of agriculture to decide if they merely want to comply with UK law, or also comply with EU law and sell to the EU.
Non-agricultural businesses in Belfast should have fundamentally the same business legal environment as those in England.
Ireland has some agricultural concerns with respect to NI, but there has been no claim that English farmers won’t be able to realistically sell to the EU.
Northern Ireland is a tinderbox. If you piss off one community too much the whole province goes up in flames, and Britain will get the blame from everyone, including the US with its sizable Irish American population. Northern Ireland already had special treatment from its inception by having its own ‘Home Rule’ assembly since it was first created. A fudge is neccessary so that the Unionists feel that their right to remain British is respected, but the Nationalists get to feel that they are part of the wider Irish nation. That is why there has been peace in Northern Ireland all this time. The NI Unionists might get a bit upset that they’re ending up in the Irish common customs union but when they see they’ll still have the Queen on the money and the Union Jack still flying over Stormont they’ll get over it, as long as someone doesn’t try to change the status quo further to the point they are forced to join the Republic of Ireland and be ruled from Dublin.