An obvious fix (for me) is to pre-process the digest, and
hyphen-escape all lines which follow the "END OF DIGEST" line and
which begin with a '-', by adding an extra '- ' at the start of line.
If I do that, then burst will do the right thing, and ignore all of
the trailer text.
Isn't it simpler to
sed '/^- *END OF [^ ]* DIGEST/q'
if you're preprocessing? If your sed has -i then you could run it on
the digest first to do the change in situ.