Our new stability policy mentions the possibility of future keywords. We should reserve a space for keywords to be defined later.

For the keywords themselves I would propose:

; reserved keywords are 2 to 8 lowercase ASCII letters
; keywords might be preceded by a starter sigil
; if we define one, it should be defined by the containing production
reserved-keyword = [2*8](%61-%7A)

For their integration, I would incorporate their use into declarations and body, but things are in flux to such an extent that we should wait for those specific changes.