I'm not a CRJ guy, but...
In my experience loss of motive flow and electric pumps would be extremely rare.
Motive flow pumps are powered simply by a small amount of high pressure fuel from the high-pressure engine driven pumps. They use a venturi setup to move a larger volume of fuel at a lower pressure. They have no moving parts, and hence are very reliable. Generally, you have to lose that high pressure feed, have a clog or have no gas in the tank for a motive flow jet pump to fail.
Electric pumps are less reliable because they have moving parts. However, on the airplanes I have flown, there are multiple pumps in each tank.
However, if for some reason you happen to lose ALL of those low pressure pumps, the engines on the airplanes I've flown will usually continue to run.
In the Sabre, when you lose the low pressure pumps (all of which are electric if I remember right, correct me if I'm wrong) there is actually a chart to tell you if the engine will still run. I think below about 25,000ft the high pressure engine driven pump will be able to suck enough gas from the tank to keep it going. Above that, the first indication you have of a failure will be the engine spooling down. Once you get down into the suction feed envelope, you can generally get a re-light.
In the E120, the engines will also keep running if you lose that low pressure feed. We actually check this on the airplane once everyday sort of indirectly during the first flight of the day checks. When we open the x-feed, motive flow is automatically shut off, and the electric pumps are used to feed the HP pumps. Well, at a couple of points, all the electric pumps and motive flow pumps are not working, and the engines keep on running.
Now, I don't know specifically about the CRJ, but I'd be willing to bet that the engines will keep running if you are not up too high. However, those low pressure pumps are there for a reason and having them all fail would be a bad thing, as the posibility for a flame out will be higher.
All that being said, remember that if you lose the HP engine driven pump, the first thing you'll notice will be the engine spooling down, regardless of how many other pumps are running, and you'll never get it to re-light. The HP pump boosts pressure by a very large amount in order to force enough fuel through the atomizers in the right spray pattern to keep the engine running.