When I've worked on trigger / sears /hammers I've just drilled holes in a steel block and placed pins in them with the same geometry as the gun, then used a fine stone and black from a candle flame to show where I've been. On some guns where the side plate comes off it can be done in situ; turn up some threaded pins and bolt them to the frame through the existing action holes.
Alternatively on page 136 of "Gunsmithing with Simple Hand Tools" by Andrew Dubino there is a diagram of how to attach a piece of steel rod to a vice that can then act as a simple stoning jig. All you do is put a threaded hole into the end face of one of the vice jaws, and cross drill a 4" piece of rod. Put a screw through the rod and tighten up using a spirit level to keep it dead flat. Arrange the hole so that the rod is about 1/16 th or so above the vice jaws.
Here's a pic of a precision filing jig for a lathe; you could easily adapt it to action and trigger work; Hemingway kits in UK sell the parts for home assembly .... I'm going to make one only a home brew version a bit bigger.

Eug