Adam,
400-500 ft MDAs ARE for non precision approaches, which intrinsically have no fancy stuff. It is very rare to have MDAs higher than this - and where they exist it is typically due to some quite nasty terrain - look at BGBW.
The key difference between operating in marginal VFR (and a 500 ft cloud base is pretty marginal) and an IFR approach is - When you are at 500ft and clear of cloud (i.e. MVFR) you know you are clear of the ground because you can see it. When you are approaching down to 500 ft, you know you are clear because you have set your altimeter correctly, you have navigated precisely along a surveyed route, or you have navigated along a route you have made up having carefully inspected a topo map to ensure you are clear about where any cables, antennas, buildings, hills, etc are.
The whole reason we have non-precision approaches is to get safely into very MVFR type conditions to visually manoeuvre to the field and land.