> I think adding it to one or two lines in a kind of signature like:
>
>  [[ view full bug history in https://debbugs.gnu.org/.../12345]]
>  [[ repliers consider keeping these lines ]]
>
> ... in the same situations that the subject line is mutated.
> Could be a simple to implement and very useful idea.

The link would be to the specific post within the
bug thread.  So instead of "view full bug history"
I'd suggest something like "View in bug thread"
or "view in context" (or "full history" instead
of "thread" or "context").

It's about viewing the particular post, but within
the full thread.