Andy wrote:

It doesn't need to, as the Boundary Zone X tickets are only supposed
to be available with season tickets and the rules say that when using
a combination of two tickets, when one is a season, the train doesn't
need to stop at the changeover point. Even if this were not the case,
there is an argument that a ticket with Boundary Zone X on it would
count as a Zonal ticket (two of these can be used in combination), as
it is potential valid by more than one route.


I think you are mistaken on the seasons bit. The examples in the (no longer
online) section K of the NFM clearly explan that a BZ to X ticket is a type
of excess, and give a number of examples of use with day travelcards.

Example:

"The holder of a Zone R1234 Peak Day Travelcard wishes to
travel from Richmond to Twyford via London.
Issue an excess fare ticket from Boundary Zone 4 to Twyford."

Paul S