Could the cult of Taal have information on the waystones? Looking at the various imperial cults around, I'd say they're the most likely to do so with their close ties to the old faith. Making a good contact with them in exchange of the possibility alone might be worth it.
Consider the history of the Cult of Taal as we know it from Tome of Salvation.Taal was patron god of the Taleutens. When they crossed into the Reik basin there were no cults or organised religion, tribal chieftains unified temporal and sacred power, being seen as being closest to their gods. Note that the chieftain was the representative of their tribes to all the gods they recognised. The chieftain of the Taleutens wasn't like a proto-priest of Taal. He would lead the sacrifice to each god on Their own holy day and he would be the one to call for Their aid. The gods of the most successful tribes proliferated and became recognised by the other tribes, with Taal recognised as the predominant god of nature/natural phenomena. For about seven hundred years into the tribes' occupation of the Reik basin there was no cult or priests of Taal.
About three hundred years before the coronation of Sigmar, the situation changed, strongly hinted to be because of contact with Tilean organised religion, priests and cults of the various gods were established. Initially priests of individual gods began taking (or being delegated?) responsibility for dealing with their particular god from their tribal chieftain. As priests became more common, priests of the same gods in different gods came together to form the Cuits to their gods. It's described that in this era the various holy texts of these gods were written with the help of the (originally Tilean) Cult of Verena. This process of cult formation wasn't always peaceful, there was clearly conflict between priests of the same god.
Once the cults were formed, there was further competition between them. The Cult of Taal decided that they weren't satisfied with Taal being recognised as the predominant nature god, they seem to have decided that he should be the nature god, so did their best to destroy the Cults of the other gods of the natural world and claim their gods was the true god of whatever it was. This is when the Cult of Söll as god of the sun was destroyed outside of Wissenland, and Cult of Ahalt as god of fertility was destroyed entirely. This wasn't unique, the Teutognen priests of Ulric exterminated their rivals completely as well.
If we can trust Tome of Salvation, this tells us that the earliest records of the cult of Taal should date to the couple of centuries before the coronation of Sigmar, when the cult itself came into existence. They may also have captured some records from the other, short lived cults they defeated
Note that this is all a discussion of the history of the Cult of Taal as the institution that exists in the Empire. It's not a history of the God itself.
What this record does make me wonder about is what the cuits of the classical gods may know, as they were organised institutions before those of the northern gods. The records of the Tilean Cult of Karnos, who the Empire believe is identified with Taal, might be more interesting for Waystone research than the Imperial version.