What do you mean by "custom bike" ... Presumably you mean a bike that was at some point a standard production bike, built by a recognised major manufacturer, with a normal VIN and associated paperwork.
No matter what it is, it has to pass this inspection:
Law Document English View See schedule 6.
The tricky bit is the interpretation.
There is nothing in Schedule 6 that discusses what standards the lights have to meet, other than requiring that they
not be marked "not for highway use".
The word "prescribed" as the first word of section 6.1 is a problem. Prescribed by what?
The words "correctly" and "properly" appear. What defines "correct"? What defines "proper"? Is a microscopic tiny LED that turns on when it's supposed to and turns off when it's supposed to, "correct" and "proper"? (I would suggest, "no".) Or does it have to be the exact original-equipment part number that was supplied by the original vehicle manufacturer with the correct original-equipment bulb in it? (I would suggest, that's probably going overboard.) Or can it be a substituted part that functions similarly - but what's that mean? DOT-marked? Or just general approximation? Or that all too rare commodity, "common sense"?
There is a performance standard for headlight aiming, but not for how bright or how dim it can be.
AFAICT the custom bike crowd have their go-to mechanics who take care of this paperwork to make the, ahem, "issues" go away.
I would have a friendly chat with the mechanic that you propose to do the safety inspection, before you commit to a course of action, and then obviously give that mechanic the business when you are ready. Start with whichever shop that you normally deal with. If that is unproductive, track down the shops that the bobber/chopper etc crowd uses. I don't want to name names, but it's not hard to figure out.