Accidentals attach to a specific figure number or replace it, using standard symbols

A sharp, flat, or natural before a figure number alters that specific chord tone; a diagonal slash through a numeral is the standard shorthand for a raised sixth, equivalent to writing 6 with a sharp. A lone accidental with no numeral implies the third.

An accidental is typed as a separate figure entry unattached to any numeral where a numeral was intended, so a renderer might stack them as two lines instead of one slashed numeral.

Gould; Read (full table of accidental-with-figure conventions, including the slashed-numeral raised-6th shorthand).

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.