Hi Jon,
If you have the Guild's Timber Framing Fundamentals book there are three scribing articles in there, the last of which details round to round mortise and tenon connections specifically. There are three main methods described: a coped joint (laid out using bubble scribers), a housed joint (where the tenoned piece has a square shoulder and goes into a housing in the mortise piece that completely surrounds it; this is similar to templated Asian methods I've seen) and mitered joint. The latter is what many log builders use in trusses, can be laid out with a laser level or plumb bob and eye, and is also described in the Log Construction Manual by Rob Chambers and I think available from the Int'l Log Builders Assn.