As per the 1963 "Holdsworthy Aides" catalog (see page 76), there were indeed 27" tubular rims available from the likes of Fiamme, Mavic, Weinmann, and Constrictor.
These rims are under the heading "Rims-Sprint."
Here is some of the text:
Fiamme had 27" road model and a track model tubular rims.
Mavic had a Piste model (probably the 27" tubulars on the current wheelset auction on eBay.)
Weinmann produced 27" "hollow alloy" and "wood lined" tubular rims.
And Constrictor produced a Conloy rim (27") of which the description reads: "Probably the strongest hollow sprint rim made, has a very deep well so that most TUBULARS need a rim tape to lift them from the edge, or the walls are liable to chafe. 27" only.
Page 77 of the Holdsworthy Aids reads: "RIMS--WIRED ON" at the top of the page and goes on to list Fiamme, the Constrictor Asp, the Brillani, and the Weinmann Alesa.
Of course there could be some part of the equation that I am totally missing here and therefore have absolutely no idea what I am talking about.
Perhaps Hilary can shed some light on this subject?
Palo Alto, CA