Has anyone heard of or have experience with a reverse pressure radon mitigation system for your home?
Background

Radon is a byproduct of radioactive decay.  You don't have to live on top of a uranium deposit; there are trace amounts of radioactive minerals in common rock, like granite.  So virtually anywhere you live, the soil slowly releases minute amounts of radon.

Outdoors, the wind disperses it.  What seeps into your basement tends to accumulate.  Your HVAC system will circulate some, and the exchange of outside air will get rid of a little.  But radon is heavier than air and basements usually aren't well-ventilated, so the basement is usually where you see the highest concentrations.

Because radon is heavy, it also tends to remain in your lungs when you breathe it, and being radioactive, it's a source of lung cancer.

The 4 pCi/L threshold isn't a magic number below which you're safe and above which you die; it's somewhat arbitrary.  Any amount carries some risk, and the government had to weigh risks and costs in order to pick a number at which they recommend doing something about it.  They may just have picked 10x the average outdoor level of 0.4, perhaps because inexpensive technology can typically keep a basement below that level, and that represents a minimal health risk compared to other things people are exposed to every day.

So if you do a radon test and it comes back 4.2, the risk to your health isn't substantively different than if it came back 3.9.  If you don't spend time in your basement, the risk to your health is relatively low.

However, you would likely need to mitigate it when you go to sell, so you might as well do what you can do cheaply now, and benefit from it yourself.

Negative pressure systems

The negative pressure systems are a pretty simple mechanism.  There is a layer of gravel under your basement slab that has air filling the voids between the stones.  The radon collects there.  A pipe is placed in that space.  The pipe contains a blower that sucks air from the gravel and vents it at roof level.  This removes a lot of the radon before it can migrate into the basement.  The pressure indicator shows the relative pressure.  As long as the pressure in the pipe going into the gravel is lower, it will theoretically be exhausting radon from under your house.

What can go wrong?

The system relies on a few things to work:


A "normal" level of radon.  The level of radioactivity in the soil varies.  If your house is located over veins of uranium, this kind of system may not be enough. 
A working blower.  Like any mechanical device, the blowers don't last forever.  If it completely dies, the pressure indicator will show no pressure difference.  But before that point, it may become less efficient, and less effective at removing radon.  If the system is old, check how much air is being exhausted.
An unobstructed pipe.  Anything that reduces air flow in the pipe will reduce its effectiveness.  Check for anything that shouldn't be there at the exhaust end.  Obstruction at the intake end will leave the pressure indicator showing a pressure difference but little air being removed from the gravel.  Water in the gravel would be an obstruction.  The gravel can also accumulate silt if there is a water problem.  The amount of air being exhausted will be a clue.
Air in the gravel.  If the foundation has a water problem (high water table, rain water collecting, broken sewer pipe, etc.), the gravel voids can fill with water.  This won't reduce the release of radon, but it will prevent the abatement system from removing it (and the pressure difference will still look like the system is working).  Dampness or water in your basement will be a sign of this.  If you have a sump pump, the basement may be dry even with water in the gravel, but that will provide a path of easy access for radon.  If there is water under the slab, it will, at a minimum, reduce the effectiveness of the abatement system.


So those are a bunch of things to check.  Beyond that, there is a general process of the abatement system degrading with age while the basement becomes more permeable to radon.  The foundation settles, concrete and mortar develop more cracks, etc.  If the basement has a large footprint, a radon abatement system at one end may be challenged in exhausting radon from the entire area.  

If there is no obvious cause for the system not achieving levels below the threshold, one solution may be to install a second system at the other end of the basement; they are relatively inexpensive.