The problem to some degree of putting a small cabin on a (smallish) rib along the lines of a doryesque fast fisher, seems to be seat height.
Like the humber on the OP, if you make the cabin high enough to make a cabin and also afford protection from the weath, you either can't see over it, or sit very high in relation to the tubes, so you feel like you would fall in easily.
bigger ribs of 7m plus appear to have enough room for a cabin with sides which extend around where you sit. such as the redbays with wraparound or full cabin on the larger again boats.
Hard boats by design seem to have higher sides, therefore you feel mentally safer in a 6m doryesque boat with a little cabin up front.
i can't really see a solution to providing much shelter on sub 7m boats. Other than the greek canopies that have arches made of tent poles to provide a taught canopy from bow to console and provide some shelter.
Just a thought for the day on my lunch break.