But this strategy using labels seems to work...

leipzig-gloss.typ

@@ -65,9 +65,9 @@
 
         if header_text != none {
             if header_text_style != none {
-                header_text_style(header_text)
+                [#header_text_style(header_text) #label("header_text")]
             } else {
-                header_text
+                [#header_text #label("header_text")]
             }
             linebreak()
         }

new-example.typ

@@ -1,6 +1,7 @@
 #import "leipzig-gloss.typ": gloss, numbered_gloss, gloss_count
+#import "linguistic-abbreviations.typ": *
 
-#set gloss(header_text_style: text(color: "blue"))
+#show label("header_text"): set text(blue)
 
 = Saimiar
 
@@ -14,3 +15,18 @@ From the song Tetrachromacy (_xoikesêcuxasø_) by King Gizzard.
     morphemes: ([three], [color-#sc[1sg]:#sc[pos]], [divide-#sc[cop]], [#sc[all]-un-see-#sc[nom]], [#sc[pot]-see-#sc[nom]], [desire-#sc[cop]]),
     translation: [my three shades are divisible, lust to see the invisible],
 )
+
+#gloss(
+    header_text: [vath sêcubai dhiakŋo, somyxasan krexasir êsuŋo],
+    header_text_style: text.with(fill: green),
+    source_text: ([vath], [sêcu-bai], [dhika-ŋo], [so-my-xas-an], [kre-xas-ir], [êsu-ŋo]),
+    morphemes: ([three], [color-#sc[1sg]:#sc[pos]], [divide-#sc[cop]], [#sc[all]-un-see-#sc[nom]], [#sc[pot]-see-#sc[nom]], [desire-#sc[cop]]),
+    translation: [my three shades are divisible, lust to see the invisible],
+)
+
+#gloss(
+    header_text: [This text is about eating your head.],
+    source_text: (text(fill:black)[I'm], [eat-ing], [your], [head]),
+    morphemes: ([1#sg.#sbj\=to.be], text(fill:black)[eat-#prog], [2#sg.#pos], [head]),
+    translation: text(weight: "bold")[I'm eating your head!],
+)