You could try Passage: it has a cap height of 7px! But mind: if you need to support non-English characters, 8x8 won't be enough.
EDIT: considering descenders, Passage won't fit an 8x8 grid without a few tweaks.
thanks for the recommendation – I already got passage!
the size is a hard constraint for pico-8 fonts :) I am used to cheating with descenders (like moving the character 1px up and deleting 1px from the descender), and for accents there’s a trick too (use a control code to overlay a separate ^/etc character on top)