Some suggestions - What you can do, although it will not show up in the name index, is when in edit mode for a person, add a custom event. For example under custom event add "served WW2". This will appear under the persons name with date of birth etc. See the person who will show in the related link as below.
Also what I did to remember the 100 year anniversary of WW1 was to list on my home page, the names of persons in my tree, who served. Also I put a poppy against the names of those who died. Click on my tree icon link, as above, to view. The Remembrance section is well down the page.
That does not cover exactly what you suggest but is a way of showing who served their country in the wars.
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