Per the quote before from Nicea about Lent, I'd suggest they did try to norm the practice. As well Schaff and you seem to assume the ash Wednesday of today began with Gregory. But what does Gregory actually say? Arguing against this is the idea that East/West were united and did follow one observance (a 40-day Lenten period), but since the East usually? does not have an ash Wed, though the West does, it suggests the ASH Wed part started subsequent to the 1054 split, which aligns back to the OP.
SO, EO and RC both have a 40 day Lent, right? But RC has an ASH Wed, while EO does not. (typically speaking) So, again, this common observance (Lent) may source to Gregory. But ASH Wed per se would only source to after 1054.
IOW, if Schaff and you are right, then ASH Wed would be practiced by both EO/RC since it sources to Gregory prior to the split.
Schaff:
"The Easter festival proper was preceded by a forty days season of repentance and fasting, called Quadragesima, at least as early as the year 325; for the council of Nice presupposes the existence of this season.733 This fast was an imitation of the forty days fasting of Jesus in the wilderness, which itself was put in typical connection with the forty days fasting of Moses734 and Elijah,735 and the forty years wandering of Israel through the desert. At first a free-will act, it gradually assumed the character of a fixed custom and ordinance of the church. Respecting the length of the season much difference prevailed, until Gregory I. (590604) fixed the Wednesday of the sixth week before Easter, Ash Wednesday as it is called,736 as the beginning of it. On this day the priests and the people sprinkled themselves with dust and ashes, in token of their perishableness and their repentance, with the words: Remember, O man, that dust thou art, and unto dust thou must return; repent, that thou mayest inherit eternal life.
NPNF2-02. Socrates and Sozomenus Ecclesiastical Histories - Christian Classics Ethereal Library