St Seraphim's probably the world's most beloved Russian Orthodox saint, much like St Francis is from Western Catholicism - many people don't know the extent of these men's beliefs and how grounded they were in their respective churches (like how loyal St Francis was to the Pope which makes Anglican Franciscans a bit oxymoronic) but they both have big followings outside those churches.
In Rome's view because Orthodoxy has never defined as doctrine anything against its own defined doctrines born Orthodox get the benefit of the doubt. In other words Rome doesn't see Orthodoxy as a separate religion or even a Protestant denomination but a Mk I version of... itself.
So Byzantine Catholics may venerate the post-schism Orthodox saints liturgically. Only a few do.
Roman Catholics can't because these saints aren't in their rite's calendar.
But anybody may privately - 'pietywise' - ask for the prayers of anybody else.
I must respectfully correct you.
There is at least one post-schism saint who is canonically included in the Roman Catholic liturgical calendar.
It's Saint Sergius of Radonezh.
Although Saint Sergius was glorified by the Russian Orthodox Church, Rome officially accepted his canonization.
Thus, a Roman Catholic Mass can be celebrated in his honor every September 25th.
How is that possible?
First, Saint Sergius is included in the Martyrologium Romanum
, the official martrology of the Roman Catholic Church. Here is a picture of the book:
On page 536, Saint Sergius is ninth of eleven saints listed for September 25th.
Here is his entry in Latin: In monasterio Sanctissimae Trinitatis in Mosquensi Russiae regione, sancti Sergii de Radonez, qui, primum in silvis asperis eremita, dein vitam coenobiticam coluit et hegumenus electus propagavit, vir mitis, consiliarius principum et consolator fidelium.
Second, Article 316-C of the General Instruction of the Roman Missal
states: On the weekdays in Ordinary Time, the priest may choose the weekday Mass, the Mass of an optional memorial, the Mass of a saint inscribed in the martryology for that day, a Mass for various needs or a votive Mass.
Sept 25th falls in Ordinary Time in the Roman Catholic liturgical calendar.
In the US, there is no obligatory memorial for that day, so any Roman Catholic priest has the option of celebrating a Mass in honor of Saint Sergius on Sept. 25th.
Thought I'd let you know.