The oldest thing I found on google (by setting a custom date range Between January 1st 1985 and January 1st 2001) even remotely mentioning glowing wounds is this
http://www.maria-rosa-mystica.de/en/newpage14 which appears to be from May 1, 1998 and while it mentions glowing wounds has nothing to do with the civil war and everything to do with Christianity.
I looked for queries such as "glowing wounds" - "Angel's Glow" - "Angel's glow" "civil war" - "angel's glow" "war of aggression" - "bio-luminescent wounds" etc and as you noticed, nothing prior to the 'science experiment' that I suspect was done entirely by the mother given what this page says http://www.americancivilwarstory.com/angels-glow-shiloh.html
>The kids were 17 year old William Martin and 18 year old Jonathan Curtis. Granted, they got some assistance from Martin's microbiologist mother, but she insists that the idea was theirs and that they performed the experiments themselves
SciShow even references that SHE was working with bio-luminescent bacteria at some point. https://nerdfighteria.info/v/7IpWJGlSSlg/
> his mother's research on glowing bacteria for the US Department of Agriculture. Phyllis Martin was studying a type if bioluminescent bacteria known as Photorhabdus luminescens, which is found in soil and glows a pale blue.
It looks like she's been retired since 2013 but she was a research microbiologist from 1981-2013 at USDA-ARS. Seems a bit lacking on evidence and just some "hey mom has this glowing thing, let's make a plausible story about how soldiers wounds glowed!"
They even try to explain the fact that they wouldn't have been able to survive on a human body (due to temperature) by claiming it's because the wounded soldiers were laying out in cold weather.