It should be scriptural because they always warn against "going beyond what is written"."
I think they mean going beyond what they've written.
I agree with your point about introducing new procedures where necessary (for example, to cover something that never existed before) - an obvious example is procedures for using video and audio at the meetings. Such things never existed in Bible times, so there could hardly be instructions in scripture about it. Likewise with modern day banking and corporate laws.
But by using a Bible phrase known to mean "what is written in scripture", they imply that procedures and processes that the GB have introduced are of equal status to the Bible. That's like the idea that what is in the Watchtower and JW publications has the same spiritual weight as the Bible itself. That's a dangerous equivalence to make, and any Christian should be very careful about that.
When looking at what the org talks about, I always distinguish between "scriptural" (endorsed by the scriptures), "unscriptural" (against the scriptures) and what I call "non-scriptural" (not in the scriptures). Something "non-scriptural", is where the Bible neither endorses or condemns it, and even verses related to the subject do not give a clear indication one way or another.
There are lots of things where the GB has either made a specific rule, or has allowed, by a process of hints and inferences, an unspoken but broadly accepted "view" to develop in the congregations, even where the Bible does not say either way. Sometimes there are even localised "interpretations" in different regions (for example, the infamous cases of brothers being counselled for not having the 'right' colour shirt, tie, hairstyle, facial hair, etc). Some of these are fairly trivial, but others are more serious and have caused a lot of anxiety and even stumbled some.