With yesterday’s update, Kern On supports manually set kerning groups. It took me a while to get it but it’s conceptually fairly easy:

• Kern On no longer erases existing kerning groups as it starts.
• As before, while KO is running, it only generates glyph-glyph pairs on-the-fly. The kerning groups are left unused.
• After pushing the “Kern On” button, you can select whether you want to re-generate the kerning groups. If not, Kern On will use your manually set groups.

https://kern-on.com/update/

Update Kern On

@kernon Say "a" and "à" are in the same group, would KO still make an exception for something like "Tà" vs "Ta" if you choose the keep groups option?
@colinmford Yes, this is how it works. It just means that the step of the process when kerning groups are automatically created is skipped and instead, the manual ones are loaded (of course, the “manual” ones may in fact be generated by Kern On in a previous session).

@kernon great! Follow up question — is there some sort of threshold for KO to decide whether to make an exception or not?

For instance if the difference between "Tà" and "Ta" is 20+ units of course I would like it to make an exception, but if the difference is ~2 units, I might prefer it did not.

@colinmford Yes, Kern On applies a threshold. While doing so, it takes into account the deviation (as you describe) as well as the frequency: infrequent pairs are given somewhat more tolerance than frequent pairs.