Gleichstellen = to treat as equal, so the idea of applying that term to a woman being legally declared a man seems right. But as for "eagrel", I also haven't been able to find anything other than the Midgard
reference (although I've only done a very brief search.
Given that the "eagrel women" are women who are treated as equal to men, and given that the relevant Midgard
sourcebook was published in 1998 and Tim Eccles first released All Quiet in Kislev
in 2002, it could be that he got the term from Midgard, if its writers made up the word (I can't speak to his awareness of a German-language RPG, so this is just a guess). That said, it could be a genuine historical term that one or both got the idea from.
Edited to add: I kept digging after I posted, and any non-Midgard
references I could find were to other WFRP fan-publications. Mad Alfred uses the term in his 1e adventure The Witch Hunter Cometh
. More fruitfully, in 3e's Averheim Electoral Crisis
, the author included this note:
The "eagrel" concept comes from earlier edition warhammer and is referenced in Warpstone Magazine, going back to before there were "Brigundians". I like it as a cultural memory making this part of empire friendlier to strong female leaders (useful to explain Nuln too).
I'm not sure if/when I'll have time to trawl through back-issues of Warpstone, but it would be interesting to know when the earliest WFRP reference to the concept is.
Edited again: I found the Warpstone issue quite quickly (good old document of contents). Issue 12, Winter 1999/2000, The Correspondent: Women in the Old World, written by (you guessed it) Tim Eccles. He mentions "aridani", "eagrel" and "gleichgestellte", noting:
I am indebted to Anthony Ragan and Florian on the WFRP mailing list for pointing to this concept in Empire of the Petal Throne and Midgard RPGs, from where the terms Aridani and Eagrel are taken respectively. Gleichgestellte is (apparently) a rough modern German equivalent.
It sounds to me like Eccles did not believe there to be, or was unaware of, a real life historical precedent for the idea.