It sounds like you are calling logs in the field on softwood, but only occasionally on hardwoods and what you want is to get board volume from only the logs called in the field? If that's correct, then I think the answer is to use Assisi's Market grading process to map field called logs to board, but anything other logs Assisi creates during compilation to cubic.
This can be done by calling both a Grade and a Quality on logs in the field. Call the Grade as usual, but also assign a Quality of say "Sawlog" as well. Then in the Assisi Market, set Min Quality to "Sawlog" for all grades you are mapping to board volume. For grades being mapped to cubic, such as Pulp, leave the Min Quality setting blank.
When Assisi compiles your cruise, the "Sawlog" Quality requirement will restrict those grades to only the logs you called in the field. Other logs that Assisi calculated and grades during compile will not have the "Sawlog" quality and therefore wont be mapped to "Sawlog" grades.
For example, here I've selected "S" Min Quality on DF grades in the Market Editor. "S" is the code for the "Sawlog" Quality