This might be possible by using the alphabet="" parameter in the shortcode:
[a-z-listing alphabet="Aa,Bb,Cc,Dd,Ee,Ff,Gg,Hh,Ii,Jj,Kk,Ll,Mm,Nn,Oo,Pp,Qq,Rr,Ss,Tt,Uu,Vv,Ww,Xx,Yy,Zz,."]
The default alphabet setting is:
Aáà??aáà?a,Bb,Cc,Dd,Eéè?êeéè?ê,Ff,Gg,Hh,Iíì??iíì??,Jj,Kk,Ll,Mm,Nn,Oóò??oóò??,Pp,Qq,Rr,Ss?,Tt,Uúùü?uúùü?,Vv,Ww,Xx,Yy,Zz
You can add any character except the comma , because that is used to separate each group. Multiple characters may be included in a single group as shown by the vowels above which have several more variants in their groups than the consonants. So if you want the $ and % to be grouped with the . then you can do:
Aa,Bb,Cc,Dd,Ee,Ff,Gg,Hh,Ii,Jj,Kk,Ll,Mm,Nn,Oo,Pp,Qq,Rr,Ss,Tt,Uu,Vv,Ww,Xx,Yy,Zz,.$%
The first character in each group is used as the title of that group. By default the first character and therefore the title for each group as defined by the list above is the upper-case unaccented letter e.g. A, B, C, etc. If I rearranged the groups to put the lower-case unaccented letter first in each group then their titles will change to the lower-case unaccented letter. I hope I’ve explained that well enough for you to see how it works, because it is a complex mechanism that I really need to improve somehow.
