Anyone out there make some sort of rear sight fixture to allow for dovetail style pistol sights to be used on a Rem 870?

XS sights

Williams make receiver sights that will will accept Williams open sight blades with pistol type notches.
These need to have the receiver drilled and tapped.

They make several types of sight bases, and most of them use the same sight dovetail, whether it's for a peep or open blade type sight.
Most of the open blades are rifle type "U" notches but can be used like a pistol sight.

The WGOS/WDOS is a "clean" sight base set:

http://www.brownells.com/rifle-parts/sights/rear-sights/wgos-wdos-bases-prod16349.aspx?psize=96

You then pick the open sight blade you want for the base.

There are other types of shotgun rear sights that may accept pistol sight rear blades.  
Brownell's will have most available sights, so you might contact their Tech Staff and ask what's available.
If nothing else they can tell you what the size is of the Williams sight bases, and they can tell you what pistol sights to fit are available.

You can mount pistol sights on the vent rib, just requires cutting dovetails. It has become a common 3 gun mod.