I cook a pork butt at 225 all night all the time.
Preheat the oven to 400, to sear the outside and kill any bacteria that might be on the roast. As soon as you put the roast in, turn the oven down to 225, and let it cook 18-24 hours.
I haven't done it with a frozen one, but I am sure it would be fine. It would be defrosted in a couple hours.
The interior of the roast shouldn't have any bacteria in it, only the outside.
That is why hamburger is a good way to get food poisoning--the exterior of the cuts, with the bacteria, is mixed all thru the hamburger. If you don't cook the burger til well done, you risk food poisoning. The same thing with whole chickens--if you don't get that interior space hot enough, the bad bugs can live long enough to make you sorry.
I just haven't been the same
since that house fell on my sister.