Twelfth Night; or, What You Will is a comedy by William Shakespeare, believed to have been written around 1601–02 as a Twelfth Night's entertainment for the close of the Christmas season. The play expanded on the musical interludes and riotous disorder expected of the occasion, with plot elements drawn from the short story "Of Apollonius and Silla" by Barnabe Rich, based on a story by Matteo Bandello. From WIKI.
In computer security, shoulder surfing refers to using direct observation techniques, such as looking over someone's shoulder, to get information. It is commonly used to obtain passwords, PINs, security codes, and similar data. Wikipedia
in loco Santaclausis
A play on the Latin phrase "in loco parentis," which means "in the place of a parent" and refers to the legal responsibility of an organization (e.g., a university) to take on some of the functions and responsibilities of a parent.
friends who warbike around town
Warbiking is, in Internet vernacular, searching for unsecured wireless networks by bicycle. 
Daytona [...] suspends here comical-Negro shtick for a minute
So is Daytona black or not? Up to this point, it seemed she was, but perhaps not? More of that Pynchon ambiguity...
I don't see this as saying she isn't black, but that she's given up on the shuck and jive persona she'd used with Maxine up to this point....
All About Eve and shit
The 1950 film is about a successful actress and her young fan Eve Harrington, who insinuates herself into the older actress's life.
A mohel is a Jewish person trained in the practice of brit milah, the "covenant of circumcision." From WIKI.
Out there somewhere in that nomad's field of indifference, riding the Chinese bus into a futurity of imprecise schedules and reduced options
This is reminiscent of the fate of Tyrone Slothrop, in Gravity's Rainbow, pages 738-742 (original Viking editions), where he is "scattered" and eventually sort of disappears from the action, dissolves.
Windust's fate echoes Slothrop's:
He's silent, wherever he is. One more American sheep the shepherds have temporarily lost track of, somewhere in the high country of this ruinous hour, cragfast in the storm.
wadis of deep purity
Wadi is the Arabic term traditionally referring to a valley. In some cases, it may refer to a dry (ephemeral) riverbed that contains water only during times of heavy rain or simply an intermittent stream. From WIKI.
Another Lawrence of Arabia reference, following from convo with Vyrva on p. 400. See Sharif's info and a photo on WIKI.
melismas of desert wind
Melisma is the singing of a single syllable of text while moving between several different notes in succession. Music sung in this style is referred to as melismatic, as opposed to syllabic, in which each syllable of text is matched to a single note. From WIKI.
a pair of fishhooks
"Fishhook" is gambling lingo for a Jack of any suit.
a music track heavy on the hijaz scale
The hijaz scale aka the Phrygian Dominant, Hijaz-Nahawand, Fraigish is distinguished by the augmented interval between the 2nd and 3rd degrees of the scale, which is commonly heard in Jewish, Greek, Turkish, Arab, Persian, and Flamenco music. The scale is: D Eb F# G A Bb C. Wikipedia
his careless gift of boy's cruelty
A "careless" gift is a meaningless gift given with little thought. It's Windust giving the Montauk Project his gift of "boy's cruelty" which was nurtured and developed. See page 339 Windust would be "a preadolescent boy [...] abducted circa 1960" by the Montauk Project, as described on page 243.
General Service 1800 series numbers are used to designate various government special agents. See WIKI.