Thanks Jim, Mike, for your responses

What you describe Mike is essentially the process I settled for. I create a static end joint model, then copy it onto the end of a timer -or rather, I copy that model, make it unique, and build a timber on its end. Then I can adjust the joint to its specifics, like width of the timber and the specifics of the two timbers adjoining it (these are stacked lap joints with a unique lap above and below each timber)

Your description of the powers of sketchup effectively answer my question, telling me I won't be able to do exactly what I'm looking for, and will have to settle for the tedious manual approach I've come to.

To be clear, Mike, when making a model to illustrate a frame or something, I do create every joint that is visible. That is, if there is a through tenon for example I do actually create that joint. I just leave timbers butted when a blind tenon is used, if I don't intend to explode the model.

A big reason for this is, the compound shapes of many timbers in the sort of work I do means that the joints are complexly shaped, this being tedious to accurately model.
the TF rubies is a wonderful tool for what it was designed for, unfortunately I find a great deal of my work to be outside of its intended use. This is not a complaint of the program, you can't expect something to perform a task it was not made to do!