Highlighting Hx?

Would it be too weird to, in addition to highlighting stats, do the same with Hx?

I don´t know... The whole group or the MC could name 2 characters, so if one of them rolls +Hx to help or interfere with the other one, he marks experience.
Or maybe a sort of inversion: if you interfere with Jammy, Jammy´s player marks experience.

It could be done in addition to or in spite of a highlighted stat. Maybe this way: "Okay, I won´t highlight weird this session, but rather your Hx with Scarlet...". The max would be 1 Hx and 1 stat, not 2 Hxs.

And now that I think about it, what about highlighting barter sometimes? Mark experience every time you spend barter (if you spend barter to roll, mark experience once no matter how much barter you spent).

Any comments?

In technical terms, that's a custom move on the home front.

I don't foresee any problem with it.

Would it be too weird to, in addition to highlighting stats, do the same with Hx?

As MC I've highlighted Hx on occasion exactly like a stat, not breaking it down to a Hx with a specific other PC, but as a general stat.  Hasn't caused any problems and pushed/encouraged players to get their PCs stuck in and invested with others.