I know several IOM-based owners and the M register accepts maintenance to Part 91 which means FAA STCs are usable as usual on an N-reg.
However, the situation has been rather fluid and I would check with their director of aviation directly.
Also, you need to be resident to do it, and even then (I gather from a resident pilot who I know well) sub-5.7T aircraft are discouraged. The various GA types which ended up on the M-reg did so in the early days of the registry.
The IOM registry validates FAA licenses/ratings into IOM ones, but only for a specific aircraft tail number. So you can fly your M-reg plane on an FAA PPL/IR etc, but cannot fly another M-reg.
The ability to keep the M-reg when you sell the plane has also varied somewhat.