I made it very clear that the principle of "equal work for work of equal value" applies only within employers at present, not across employers (...)
When you wrote the above, were you thinking of an employer as being an immediate supervisor in the same physical location as all employees supervised by the supervisor?
I don't see how you get any particular degree of geographical concentration out of the concept "same employer." For example, an organization might include a number of facilities near each other within the same city or town (such as a chain of small restaurants or copy shops on a University campus or a system of library branches on a University campus). It might include a number of facilities in towns far from each other, but all within one state. It might include facilities in several states and no more than a few in any given state. It might include facilities in different countries.
and that there really is no way that it can apply across employers, not because of labour market issues, but because of all the other issues involved in determining the value of any work to one employer as compared to another employer.
What kinds of issues do you have in mind? Is the degree of geographical concentration an issue?
One final issue for this message:
The notion of a principle "applying" or "not applying" seems to be a matter of decree. If you don't intend to take on an authoritarian role, then perhaps you could formulate a particular principle very clearly and give a name to the text that formulates it. Then the principle itself (to be invoked by referring to a reasonably short name that identifies both the principle and the text that formulates the principle) will provide all the information necessary for people to determine for themselves what it does or does not apply to.
It seems that various (e.g. religious) institutions have imposed very specific rules in practice at any given time and used vague principles to justify imposing those rules. However, they didn't identify any particular person within the institution and explain that they were merely providing that person's interpretation. They insisted that the principles themselves demanded the specifics. When they wanted to revise the rules, they pretended that there was no change. They pretended that people were confused and needed help understanding "how the principles apply."