I think we should define in LDML, that for all locales "xx_Yyyy":
if xx_Yyyy is the default content, the parent is xx.
if not, the parent is root.
Then the current line in supplemental data is simply an expression of that (limited to current locales):
<parentLocale parent="root" locales="az_Cyrl bs_Cyrl en_Dsrt ha_Arab mn_Mong pa_Arab shi_Latn sr_Latn uz_Arab uz_Latn vai_Latn zh_Hant"/>
And we should have the unittest that:
For every locale L in CLDR of the form xx_Yyyy:
L is in parentLocale[@locales=] iff and only if L is not in defaultContent[@locales=]