Most renaissance and baroque scores use specific symbols for bass figures, particularly for augmented sixths with a backslash striking only the upper part of the number six (or sometimes the number nine), and "plus" symbols integrated with the number four (and sometimes the numbers two or five as well; this notation may also be used for integrating flat and natural accidentals).
Here is a way to obtain such symbols in the printed output whilst keeping the usual syntax for figured bass entry. This is done not with native MetaFont glyphs, but by combining PostScript code (using Scheme syntax so that SVG output is supported) and additional stencils in markup definitions.