Finding a way for my mother-in-law to use her wheelchair on rough terrain?
Have you thought about trying some sort of seating arrangement with springs build in to absorb shocks and/or additional cushions to offset her discomfort when in the forest trails?
Wheelchairs equipped with oversized balloon-type wheels help provide stability and maneuverability, all terrain wheelchairs can easily travel over a multitude of surfaces. Possibly absorbing shocks as well.
Example of an oversized balloon-type wheel.
Other options could be available if desired also.
The next best thing I could think of is to try out something like a X8-Extreme All-Terrain Power Wheelchair.
The Extreme X8 is the ultimate 4 wheel drive power chair. Primarily an outdoor chair, the X8 will take you through the softest sand beach to the rugged woods, dry creek beds and snow. The patented passive steering system and 4-1,000 watt allow you to go where other wheelchair users have never gone before.
With some luck and a some research you may be able to find a company that would let your mother-in-law road test it for a day on some possible envisioned terrain she would like to explore.
The tires look more absorbent to shock due to rough terrain. It is obvious that some land formations will not be suitable with this device. One could add additional cushions to ease the back pain of those with chronic back sufferings.
I would actually get the person interested in using such a wheelchair to get the appropriate approval of health care professionals first. They may also be in a position to let people know of where they can get grants to offset the cost. They may also be in a position to let you know where you can rent such equipment.