That sounds as if it shouldn't be any problem. The problem is with extensions that come from extensions wired
before the master.
There would be a problem if the main incoming feed also joined the extensions at the white box and they went together to the master. But as long as the incoming feed is direct to the master, with the line to the white box separate, no problem.
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Moved (with trepidation turned relief) to BT Infinity 2 for upload speed. Happy BE user for several years.