Derby,
You might look at this discussion again Wild callery pear rootstocks - #10 by Derby42. The idea with these seedlings is they are disease resistant and prolific but produce pears the size of peas as your aware. The seedlings may not be compatible with every variety of pear but other rootstocks such as OHxF87 are much more compatible. Since OHxF87 and others are more compatible than the wild callery pear but still compatible typically with callery they make a nice bridge for callery with a variety such as Bartlett. So you have callery roots and stem , then a piece of OhxF87 grafted on callery , then Bartlett grafted on ohx87 as an example. You can use that same technique with many pears that are not compatible with each other. As an example to clarify interstems take winter banana apple. The winter banana apple is so compatible with other trees it can actually be grafted on pears with great success. The idea of using winter banana apple is not for pears though it is to graft two incompatible apples to each other. If you have the time to read the technical answer there is more available here http://www.hort.cornell.edu/expo/proceedings/2015/tree-fruit/Understanding%20graft%20incompatibility[1].pdf
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