If I recall correctly, there are scattered references in Greek literary sources of citizens getting together and practising their drill. This was not a frequent occurrence, though, and other sources dismiss the need for training altogether. Stabbing with a spear comes naturally: the rest is down to courage and strength, they seem to feel. Anything else is unnecessary. (Source: "Greek Warfare: Myths and Realities" by Hans van Wees.) That was probably an uniquely Greek (or even Athenian) attitude, though, and I doubt it lasted beyond the Peloponnesian War. It certainly would not work for phalangite training.

Incidentally, although the farmer-hoplites did indeed leave fieldwork to slaves if they could afford it, they would still need to be present to supervise the work.