I would like to migrate dependency on ArcPy to grass.script without requiring the full installation of Grass. Would it be technically possible to have a portable directory containing grass.script and all the libraries it depends upon?
Generally yes - you "just" need to figure out the dependencies. The GRASS user mailing list may help you in case of troubles.
PS: Please post your results to http://grass.osgeo.org/wiki/GRASS_and_Python then for the rest of the world. Thanks!