Hi I am wanting to know if doghouses were ever streamlined?
No. The closest you get to that were the tenders on some N&W Js late in the steam era.
Also what were the uses of them?
The doghouse was a place for the head brakeman to ride on a freight train.
Could they have been retrofitted to act as a cab for multiple unit diesel control?
The control stand would physically not fit into the doghouse of a steam locomotive tender. Also the doghouse became quite inhospitable during weather extremes.
If you do a google search for steam locomotive doghouse, you may find a thread on this forum from several years back.
As far as I can see you have 1 locomotive that was modified in the 1970s to do what you are trying to accomplish. This is adequate excuse to model this on your railroad if you wish. The reason people keep telling you that it wasnt done is because it wasnt done in the era youve stated. You asked for prototype information and youve gotten it over and over again.
I have not seen any concrete cited resources that says this was a widely used practice.
As for a feasibility of this on a model is concerned: What system are you planning on running your railroad with and which manufacturer are you going to purchase your SP GS-4 and Alco PB from? <- This is an important choice because it effects the feasibility of what you are trying to do with the model. Also as far as I know only one manufacturer makes a PA in HO (the make a PA/B set). If you run a GS-4/PB/PA consist there is nothing to figure out.
As others have stated it is unlikely that you would find a lone PB with out its A unit. If the railroad bothered to purchase the B unit, it was because they needed the extra power to haul the assigned train.
Rule 108: In case of doubt or uncertainty, the safe course must be taken.