lay'tel SIvten ('ej latlhpu')

pa' SoHtaH pagh meyrI' SoHtaH!
"Be there, or be square!"

I don't think that grammar-wise the {pa' SoHtaH} can stand for "be
there !". Even if one speaks this phrase coloring his voice with an
imperative tone, then again the meaning would be "you are there", with
the context of "you are being there" (meaning I am making an
observation that "you are there"). And all this with an imperative
tone, which doesn't make much sense.

If I was to translate the "be there !" in klingon, I would choose
something like {pa' yISaH'eghmoH}.

If I were to temporarily accept meyrI' as meaning the same thing as the obsolete English slang meaning of square, I would pattern this utterance on existing Klingon:

pa' SoHtaHbe'chugh vaj meyrI' SoH
if you are not there then you are a square

Using pa' bISaHbe'chugh is also acceptable.

If I were to look for an actual bit of Klingon slang, I might say one of these:

pa' SoHtaHbe'chugh vaj bInatlh
if you are not there then you consume (reprehensible, disguisting, contemptible)

pa' SoHtaHbe'chugh vaj Hom SoH
if you are not there then you are a bone (weakling, runt, scrawny one, skinny one)

pa' SoHtaHbe'chugh vaj bolwI' SoH
if you are not there then you are a drooler (traitor)

And so on.

-- 
SuStel
http://trimboli.name