I understand now what it is you want Paskal, and I hope you find it.
On a small note – there is not a lot of surviving written Welsh from the period you are talking about, but:
The earliest written Welsh is a probably a stone inscription from the eighth century – Tywyn Stone.
Earliest continuous Welsh text is the settlement of a land dispute between Tudfwlch son of Llywyd and
Elgu son of Gelli, in the Book of St Chad in Lichfield Cathedral Library. The book is eighth century but the Welsh is a probably ninth century addition.
There are other Welsh glosses in Latin texts in the ninth and tenth centuries.
The twelfth century book of Llandaff holds written Welsh records in an Old Welsh linguistic style which are either copied from earlier written sources or reflect a memorised archaic linguistic form.
The Mabinogion tales similarly – although our current earliest copies are circa mid fourteenth century – freeze an earlier archaic linguistic form, probably c1100.
So there may not be a lot of surviving written Welsh from the period we are discussing – but there was obviously some about.
PS – back to my point, if the majority of written Welsh thought was in Latin, court works etc, that ties Welsh elites even more closely into the standard European elite model, bound by the language and customs of clerics in spiritual and secular roles.