I've looked in the lilypond documentation for ornaments like these
<http://lilypond.1069038.n5.nabble.com/file/n148407/shake.jpg>
<http://lilypond.1069038.n5.nabble.com/file/n148407/backfall.jpg>
The images are from the explanations of ornaments in Purcell's "A Choice
Collection of Lessons", 1696 (on imslp.org). There are a few others that
are common in Purcell et al, and the shake // (and a similar ornament with
only one line) is very common in Byrd and the other virginalist composers.
If they're not in lilypond, would be good to have them. I can't offer any
lilypond expertise, but could sponsor them if the sponsorship system is
still in place.
David