Ok, sorry for my learning curve. I see the question is whether vmkfstools can output to stdout. Has anyone tried it?
I completely second your question, and have another one,
by default, gzip is used, but would it be possible and/or useful to use bzip2 or lzma instead of gzip in order to gain space on the target ?
Otherwise, if there is enough space on a local datastore, maybe copy/compress on a local datastore, then move to final target afterwards ?