How are artificial constructions such as l33t classified with regards to English?

It's a kind of argot, jargon, or insider's lingo.

argot — An argot is a secret language used by various groups — e.g. schoolmates, outlaws, colleagues, among many others — to prevent outsiders from understanding their conversations. The term argot is also used to refer to the informal specialized vocabulary from a particular field of study, occupation, or hobby, in which sense it overlaps with jargon.

jargon — specialist language: language that is used by a group, profession, or culture, especially when the words and phrases are not understood or used by other people

lingo — set of specialized terms: a specialized set of terms requiring to be learned like a language

I'd just say l33t is an informal dialectal transcription format, not dissimilar to txtese, chatspeak, etc.

It should be appropriately grouped in with simplistic encryption methodologies, along the same lines as Pig Latin. The wording is rarely dissimilar to standard English, albeit poorly written.

It's almost more of an orthography than argot. It has characteristics of both, but if you think about its spoken form it's really not much more than slang. The written form is very different, though, so that suggests that it's primarily a way of writing.

DivinusVox is correct in relating it to Pig Latin and other forms of light obfuscation.