When shooting sports, I use code replacement (CR) from unused IPTC fields, because it's easier to enter there precisely for the reasons you stated, combined with variable evaluation (both are insanely powerful Photomechanic features, and everybody should learn about them!)
Therefore my caption might read something like
"{suppcat1} and {suppcat2} during the..." where I enter the replacement into the two supplemental category fields, and it gets autopopulated correctly.
Only problem with that approach - your system might use these fields for something already, your agency might use them for something already or might have a problem with them being prepopulatech (better check). I guess you could use
Group Label 16 through 20, I am not sure they are ever used for anything except perhaps some specialised agency label, though I might be mistaken.
I used supplemental category since back in the days of hardwired IPTC stationery ("classic"), these fields were visible and I did not have any use for them. Now, that you can for several years
customise your IPTC info and stationery dialogues (and if you haven't, you should!
) , you can easily put and use the group label 16-20 in your default IPTC dialogues (it's in Preferences/Accessibility/Customise...).
Or, if the Group label is needed (or would interfere with IPTC processing on the agency's side), perhaps file a feature request here in the forums for several
Custom labels to be added to the dialogue (and appropriate Variable to go with them). These Custom labels would NEVER get written to the file (and a appropriate warning would be writtren in it's description in the preferences), and would be used ONLY for writing the replacement codes and their evaluation elsewhere in the Caption field or wherever via variables. How about that
?