JM: Following Arons' tip I found two indications about it, both linked to the razor "Gillette":(a)  Chin Strap - Gillette.com :La barbe en collier est essentiellement une ligne de barbe qui continue les favoris www.gillette.com/glossary/fr-CA/chinstrap.shtml ; (b) Gillette | Comment se raser Découvrez les trucs et astuces de Gillette pour savoir comment se raser. ... Un collier de barbe est constitué de longues pattes qui se rejoignent sous le ...www.gillette.com/howtoshave/fr-FR/index.shtml -   

I had surmised, at first, following some of the the Nab-L discussions, that the Newport Frill was a kind of collar, as those lacy frilled ones that had been favored in Shakespeare's days and omnipresent in Dutch paintings, which are still used by the clergy in certain countries.
The reference to a specific style of trimming a beard, and to specific razors, is all too clear in Shade's lines.

I wonder why no one has pointed out that Abraham Lincoln wore a "collier de barbe." Perhaps Nabokov was aware of this in the same way as he much have known that, in contrast, in Flaubert's pen it indicated mediocrity.

I would favor a stress on Lincoln who, as Marat bleeding in his bath, was the victim of a murderer. Nabokov would have known about Shade's impending death, but he was unprepared to his father's assassination (one of the the themes present in "Pale Fire" cf. Priscilla Meyer). Nevertheless, the convoluted reference suggests to me that there must have been someone closely associated to Nabokov, in real life, sporting this kind of beard (my Dutch grandfather had a "Newport Frill" - and a pair of ear-rings to boot...).