Post updated to provide context for anyone not coming here from the NLM comment thread.
In response to a post on the New Liturgical Movement a commentator asked this question:
I just saw some pictures from a Corpus Christi procession at an FSSP apostolate in France. There were three additional priests in attendance. They were all wearing chasubles over surplices (not albs). I can’t recall ever having seen this done, and I was wondering if anyone here can offer an explanation. Could it be because they lacked a sufficient number of white copes, and if that’s why, is there a rubrical provision to allow chasubles in place of copes where copes are not available?
Gregor points out:
Under the rubrics of the usus antiquior, the canons of a cathedral chapte wear for the Corpus Christi procession the vestments corresponding to their rank in the chapter (the so called canonici parati, much like there are Cardinal deacons, priests and bishops, who used to wear the corresponding vestments for certain papal functions).
Having just looked at this question (and many, many others) in preparation for the Corpus Christi procession at Holy Innocents, I could point right away to the citation on pg. 388 of the 15th edition of Ceremonies of the Roman Rite:
“If the cathedral chapter assists, the canons out to wear vestments of their three orders; that is, subdeacons in tunicles, deacons in dalmatics, priests in chasubles; dignitaries in copes.(31) These are put on after the communion of the Mass and should be worn immediately over the rochet and an amice, without stole or maniple, as when the Ordinary sings solemn mass. The colour of the vestments is white. If the cathedral chapter is not present the clergy may be divided into groups wearing these vestments. (32)” [My emphasis]
Footnote 31: C.E., II, xxxiii, 5.
Footnote 32: Cf. S.R.C. 2362 Section 1.
That second footnote also seems to drive the wearing of albs instead of surplices as one might expect in place of the rochet. S.R.C. 2362 is now easily available via Google Books. Here’s the relevant excerpt from the Decreta authentica.