An awful lot is, IMO, wrongly inferred from obscure listing requirements and trade practices.
I see no problem with placing a KO anywhere on a NEMA 3R enclosure. Even the choice of connectors is indirectly influenced by the 'dry location only' limitation place on 'ordinary' connectors.
Let's look at the genesis of the misunderstanding:
The first part is the nature of the NEMA 3R test: 'rain' is directed at the enclosure at a slight angle for a short time, then the enclosure is inspected for 'objectionable' accumulation of water. In simple terms, NEMA-3R requirements can be met with an umbrella- but a bucket under that umbrella might fail. That's why so many NEMA-3R enclosures have drain holes or open seams at the bottom. (That's also why installing a NEMA-3R box upside-down is such a concern).
The UL standard goes on to specify that there will not be any pre-punched KO's above the bottom edge of any 'live' parts. That's why NEMA-3R panels have KO's only at the bottom edge. It was judged by UL that pre-punched KO's were likely to be damaged / opened over time, witout being used for wire entry, thus allowing water to freely enter.
Such a fear does not apply to a deliberately made opening. Nor is the possible entry of minor amounts of water seepage considered a concern; that's why 'compression' fittings were assumed to be adequate for wet locations, for so many years, without requiring actual testing.
Again, trade practice took this a step further. Recognizing that water entering from the side is likely to run down the side of the enclosure, while water from above is likely to drip onto live parts, we use Myers' hubs and other, more substantial fittings on the tops of NEMA-3R enclosures. We protect the 'umbrella.'
Mind you, this picture changes if you have a NEMA-4, Nema 7, or NEMA-12 enclosure. Those are gasketed enclosures, intended to be sealed, and will accumulate any water that enters, no matter the source. Eventually the water level WILL reach the live parts- so extra efforts (including perhaps conduit seals) might be needed to keep water out in every circumstance.