Currently it's not possible to override portage.const.EPREFIX except by using the PORTAGE_OVERRIDE_EPREFIX variable, so API consumers are effectively constrained to a single value. It would be better if there was possible to simultaneously create multiple configurations with different EPREFIX settings. For example, in the unit tests, ResolverPlayground could use this instead of having to modify the global portage.const.EPREFIX variable.