The overseas quota could only be an agreement between the clubs because it could never be enforced as a rule, the Kolpak rule deals with player from EU and countries with trade relationships but if someone wanted to challenge the overseas player rule for being racist they could have a case.
"racial discrimination" shall mean any distinction, exclusion, restriction or preference based on race, colour, descent, or national or ethnic origin which has the purpose or effect of nullifying or impairing the recognition, enjoyment or exercise, on an equal footing, of human rights and fundamental freedoms in the political, economic, social, cultural or any other field of public life
racial group "any group of people who are defined by reference to their race, colour, nationality (including citizenship) or ethnic or national origin"
Quoted the definition for racial discrimination and a racial group and underlined the obvious point which could case the overseas quota to be classed as racist. A lawyer could probably find 100 more reasons.
Just to note I am not implying the RFL are racist of that anyone who supports the overseas quota is racist I am just pointing out how legally it could be challenged.