I think another reason the electric fuel pump is wired into the oil pressure circuit is so that in an accident, if the line is ruptured (and the engine stalls) gas won't be pumped all over. No oil pressure, no power to the pump. Good idea on the tractor too.If you buy a small inline electric fuel pump rated for an import with a carbed engine you'll be fine..
I used an old AC fuel pump I had lying around that is big enough to handle a 400+ V8,looks like a mini beer barrel, on my 20 HP Kohler and it hasn't flooded it once,and it puts out 6-8 psi..the lower pressured import style pumps will put out 3-6 psi..the needle in a car carb & its float aint much different than a small engine,I wouldn't worry about it flooding unless the needle and seat needs replacing anyway..
The type of pump I'd look for resembles a small electrical transformer,they have a nylon ball inside thats operated by a solenoid..they are tiny enough to muont easily in tight engine compartmemts,but remember,electric pumps "PUSH" fuel a lot better than sucking it,so its important to mount them close to the tank if possible--I think thats why many fail rapidly,and they get a bad rap,they get mounted close to the carb and run too long airbound,and overheat and seize..
I wired mine to a push button and only energized it to prime the carb for a quick start after long sitting time--the stock vacuum fuel pump seems to work OK once its started,it can draw gas through the electric pump even with it "off",so I only use it when it has sat before firing it up--but if you bypassed the vaccuum pump you could run exclusively off the elecrtic pump OK..use an inline fuse and a switch though,for safety--the "correct" way to wire an electric pump up is to incorperate it in with the oil pressure sending unit ,so it wont keep running if the engine stalls,to avoid filling the crankcase with gas--but I haven't gone to that extreme on my setup,since I use it mostly as a "primer" to fill the carb bowl up after its sat a long time,not constantly..
I like your idea of a 'helper' pump...