The automatic dtls feature should add the following mappings if the corresponding .dtls glyphs are missing from the font (provided that the target glyphs are in the font, off course, and ideally it should do so by checking for the relevant unicodes since most math fonts don’t use nice names):

sub i by idotless;
sub j by jdotless;
sub iitlaic-math by idotlessitalic-math;
sub jitlaic-math by jdotlessitalic-math;

Ideally the font would also include .dtls variants for other math i and j glyphs that don’t have encoded dotless versions. There might also be .ssty variants as well e.g.: i.ssty (or i.ssty1 and i.ssty2) and idotless.ssty etc.

sub i by idotless;
sub j by jdotless;
sub iitalic-math by idotlessitalic-math;
sub jitalic-math by jdotlessitalic-math;
sub iitalicdoubleStruck by iitalicdoubleStruck.dtls;
sub jitalicdoubleStruck by jitalicdoubleStruck.dtls;
sub ibold-math by ibold-math.dtls;
sub jbold-math by jbold-math.dtls;
sub ibolditalic-math by ibolditalic-math.dtls;
sub jbolditalic-math by jbolditalic-math.dtls;
sub i-fraktur by ifraktur.dtls;
sub j-fraktur by jfraktur.dtls;
sub iBold-fraktur by iBold-fraktur.dtls;
sub jBold-fraktur by jBold-fraktur.dtls;
sub idoubleStruck by idoubleStruck.dtls;
sub jdoubleStruck by jdoubleStruck.dtls;
sub isans-math by isans-math.dtls;
sub jsans-math by jsans-math.dtls;
sub isansbold-math by isansbold-math.dtls;
sub jsansbold-math by jsansbold-math.dtls;
sub isansitalic-math by isansitalic-math.dtls;
sub jsansitalic-math by jsansitalic-math.dtls;
sub isansbolditalic-math by isansbolditalic-math.dtls;
sub jsansbolditalic-math by jsansbolditalic-math.dtls;
sub imono-math by imono-math.dtls;
sub jmono-math by jmono-math.dtls;
sub iscript-math by iscript-math.dtls;
sub jscript-math by jscript-math.dtls;
sub iboldscript-math by iboldscript-math.dtls;
sub jboldscript-math by jboldscript-math.dtls;

I think .ssty variants can be ignored and ssty feature made to be the last feature.

There are not that many features that are used in math fonts. They are flac, dtls and ssty. flac affects accents over capitals (something like *.case accents), so there is not interaction between it and dtls, so this leaves ssty which should be last. So I’d put dtls right before ssty.

sub i by dotlessi;
sub j by dotlessj;
sub uni1D456 by uni1D6A4;
sub uni1D457 by uni1D6A5;

I renamed the glyphs to Glyphs names for testing, and I got:

sub iitalic-math by idotlessitalic-math;
sub jitalic-math by jdotlessitalic-math;

but still no sub i by idotless; nor sub j by jdotless;. (for my actual project I can’t used the nice names for legacy reasons, so I had hopped glyphs would handle these by the Unicode values not glyph names).

I’m collaborating on a project with someone else and renaming glyphs en mass will likely cause unneeded disruptions, so I’m trying to avoid this as much as possible.

We need also to auto generate languagesystem math dflt; to fonts that has any of ssty, dtls, or flac features.

I also notice that dtls will be added now to any font that has idotless or jdotless, this is probably not needed for most fonts, so may be autogenerate the feature only of there is at least one .dtls glyph?