I saw this topic when it was first posted but decided the same as you, keithl, that I wasn't sufficiently qualified to provide an answer. However, as to whether or not one can diverge from track sufficiently to get within the 60 degree sector (30 degrees either side), Pans Ops protects for this. However, if it's not in the State AIP (or regulations, or equivalent document) then you can't do it. Period.
This might be because the particular State has not bothered to apply the necessary additional protection area to accommodate it.
As for the matter of building your own approach in the FMC, that is not permitted here. And I'm pretty sure that nobody else permits it either. However, having said that, in the next couple of months, I'm hoping to run a trial that aims to allow this in a very limited way.
It will be based on the use of a GNSS receiver, manually configured to 0.3 NM HIL. If the trial proves to be successful, we will need to rewrite a few of our regulations to allow it. That process will be lengthy, so I don't expect it to come to fruitition much before the end of the year.