It certainly sounds as if the changed hood means that if you want to change the filter or add a filter you have to remove the hood. If this is the case then with a wide angle 45mm lens a non Pentax petal type might be best rather than the Pentax round rubber hood and a lot cheaper as well.

I have no experience of petal hoods but while it seems that the large petals are positioned at the sides and the smaller petals at top and bottom, it is possible with a locking ring type of petal hood to move the petals to another position if the sun's position dictates this so that type of petal hood has advantages.

However as I said, I have never used a petal hood so I am interested in other P645 user's experiences or I suppose other 645 camera users' experience as I presume that the same considerations apply with any 45mm lens on any 645 camera