Emblustricate

Meaning:  To bewilder. Cotgrave borrows some synonyms from incornifistibulating to define the French version:

To trouble, blunder, or or pester the mind with, to beat the braines about.

Usefulness: 1 (It just sounds right: "I"m just so emblustricated!" "Your emblustrications won't work this time, my friend." "I managed to emblustricate the boss enough that he just agreed to my pay rise!")

Logofascination: 1 (Even if it weren't a 0-degree word, the OED etymology note would improve its rating: "Whimsically formed to render the equally fantastic French emburelucoquer." We who are logofascinated salute you, oh distant lexicographer.)

In the wild: not really.

Filopendulums

Meaning: Frame glosses this as counterweights which, in context (below), seems about right; something like the weights in a clock, perhaps.

Usefulness: 3 (I'd be impressed if you could work this into conversation, but Rabelais' line might be worth learning.)

Logofascination: 2 (From the Latin for thread and hanging, I wonder if Rabelais is suggesting that these counterweights are flimsily suspended).

In the wild: Although filopendulums isn't defined anywhere, the botanically-inspired filipendulous was recommended by the OED today.

Here's a wonderful word for you: filipendulous, adj.: hanging or having the appearance of hanging by a thread.
— The OED (@OED) April 16, 2013

Celivagous

Meaning:

Cotgrave: Heaven-faire*; heaven-affecting; wending, or bending, towards heaven. 

The Inky Fool: heavenward-wandering. 

Usefulness: 1 (Either in the spiritual sense, as Rabelais, Cotgrave and Sir Thomas use it, or in the sense used in The Horologicon** of a skyward glance to assess the weather. It's not that useful in Melbourne, where the state of the sky has no correlation to the state of the weather five minutes later.)

Logofascination: 1 (One of the lovely -vagous family of wandering words, as previously mentioned in circumbilivagination.)