Take not that this dangerous in an accident du to the rising angle of the belt to the shoulder strap.
Because it's a fair angle off horizontal there's a large vertical component to the tension. If a crash throws you forward, there's a massive amount of force pulling you down by the shoulders and crushing your spine.
If you can't do it properly with a bar, personally I wouldn't do it at all, unless you can use the upper seat belt mounting brackets, they would be perfect.
Even well fitted harnesses kill people in crashes. That's why Hans devices are mandatory across almost all Motorsport.
I had a long and interesting discussion some years back with a neuro surgeon who does a fair bit of decent level amateur Motorsport. He was of the opinion that fitting harnesses without a Hans could potentially be more dangerous than wearing a standard seatbelt. Inertial seat belts fitted as standard allow the energy of deceleration in a crash to be dissipated through your torso rather than just your neck and base of skull when wearing just a harness.
It was the reason why I never fitted them to my car.
Depends on the nature of the crash but yeah, constraining your shoulders just leaves your head to whip back and forth in a longitudinal impact and possibly snap some vertebrae if you do it well enough!
That said, I had a 4 point and bucket in my old mx5 daily driver. I rolled that a couple of months back and walked away completely unharmed. I'd have done much worse if I wasn't wearing the harness, but if someone had shunted me at traffic lights it could have been a different story!