Marquee rules basics only apply to English Premiership Clubs, are weird, and there seem to be so many exceptions that they effectively nullified.
Far more importantly in this case, is what you need to work. AFAIK, if you have: a British or EU or Kolpak Passport, or even a non-tourist/student Visa independent of a Rugby Career (say for example, his partner has accepted a position in the UK, and has a 3 year temporary Visa, he would be entitled to work as the Spouses, including as a Pro sporting player), the RFU and/or British Immigration can do nothing if a club wants to recruit you.
SRU is a little different as they and the two provinces are effectively the same bodies from a business POV, and so are always able to veto recruitment.
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