OK, Doug's post inspired me to share my coping jig. While it is not as dead-on stable as bearings, it does a nice job. It is easily adapted to router tables.
I have 2 of these and they have held up and functioned very well for over a year of daily use. I used UHMW plastic for the runners and by countersinking the holes, the tightness in the slot can be adjusted by the tightness of the mounting screws. Also underneath are a series of Neodymium magnets between the runner and the inside edge of the sled to keep it flat on the cast iron table.
Here is a shot of one sled on its shaper. The fence with the pair of toggle clamps on it is permanently attached to the sub base, more or less perpendicular to the table slot. The 2 bolts sticking out the back side are the fine adjustments for square. Generally, if the runner gets tightened, the squareness is recalibrated and locked in.
The front of the jig fence shows how the adjustment bolts come through to contact the backer strip. By using a small toggle for the backer strip, you can change profiles easily. Just press the backer against the fence between the 2 contact points and it will stay square. If your cutters allow tenon end trimming, as mine do, you can set the shaper fence to remove a whisker from the tenon ends. Then you can release the backer toggle and slide it over to the fence every dozen parts to keep the backer profile nice and crisp.
Here is a shot with the backer in place. I mill a few backer strips at a time and keep them near the shapers, ready to swap out as needed.
Finally, here is a rail in place, ready to cope. I took a small partial cut to confirm that the cutter is at the right height for this style. You can see the slivers that remain on either side of the groove, showing that the tenon is centered.
As I have time, I'll try to get some pics to document the sicking shaper setups as well.