There are a decent few such pieces in this collection which can be used as postludes, though they're certainly on the simpler side.

https://imslp.org/wiki/Der_katholische_Organist_im_Hochamte_und_Requiem,_Op.54_(Diebold,_Johann)

Serviam, that is a cool one!

Bombarde, anything really, just looking for people to list what they know.
I know people that really want a postlude on these tunes probably improvise it, but I don’t have too much imagination for improvising :/

There is a lot of relatively easy organ postludes by Protestant publishers based on hymn tunes, but less based on chant and even less on the Marian chants

Basically every Magnificat alternatim setting has at least one or two versets that work well. Hieronymus Praetorius, Heinrich Scheidemann, Jehan Titelouze, Alexandre Guilmant, etc. And of course there's the Bach Magnificat Fugue, BWV 733. The first and final versets of the Grigny Ave maris stella can also be very nice. Barbara Owen's early American organ music collection also has a simple but nice set of variations on O Sanctissima by Benjamin Carr.

I second all that Chris said in his post above. Those are all wonderful sets.

I might also add that there is a STRIKING setting of the solemn Salve Regina by Olivier Latry that is available.

Also, Dupré has a nice choral setting of the Solemn Salve as it is set in his Choral and Fugue, Op. 57

Basically every Magnificat alternatim setting has at least one or two versets that work well. Hieronymus Praetorius, Heinrich Scheidemann, Jehan Titelouze, Alexandre Guilmant, etc.

including Dandrieu, several Magnificats

Pieces based on the Ave maris stella:

The last verset of Titelouze's setting can be played a postlude.

Guilmant's "Sortie" from the second book of his L'Organiste liturgiste, op. 65, is also a good postlude.

The ninth piece in Marcel Dupré's great op. 18 (Vêpres du commun des fêtes de la Sainte Vierge / 15 Pieces Founded on Antiphons), based on the Ave maris stella, makes for a spectacular postlude. Not an easy piece, though.

Postlude on Immaculate Mary - K. Frecker https://www.sheetmusicdirect.com/en-US/se/ID_No/1037719/Product.aspx

VI. Postlude on LOURDES HYMN https://imslp.org/wiki/Pro_ecclesia%2C_Op.46_(McGrath%2C_Joseph_J.)

Ave Maria - Richard Clark, from Gregorian Impressions https://www.jwpepper.com/Gregorian-Impressions/10956833.item

It's a bit of a stretch...but you can play any of the mvrs from Widor's Symphony Gotique based on Christmas chant.