'promography': because 'pronography' and 'pormography' were already taken.

yet out of this, a greater purpose?

Prom is short for "promenade", a borrowing from the French derived from the Latin "prominare", meaning "to drive forward".  To "promenade" in the original sense was simply to go for a walk, though when it appeared in English around 1567 it especially meant to walk to and fro in public view in order to display oneself or one's finery.  By the late 19th century, "promenade" was being used to mean a formal ball or gala at a school or college, and it was shortened to "prom" around the same time.  (citation)

promo is short for "promotion(al)", from "promote", a 14th century borrowing from Middle English derived from the Latin "promotus", past participle of promovEre, literally, "to move forward", from pro- "forward" + movEre "to move". (citation)

-graph(y), from the Greek for "writing".  The 'first' letter of a suffix may depend on the ending of the root word to which it is attached; ergo, while the suffix is generally -ography, it may also be -agraph, as in "paragraph", or -egraph, as in "telegraph". (citation)

therefore, promography might be identified as
a class of writings
about walking around,
about the prom,
and about moving forward.

promography  is open to your contributions,
so dust off those blurring reminiscences of
your prom, that time you hiked around Europe,
and the moment when you took that first step.

Images from meaning.com circa 1999.