One of the reasons organisations like the CAA try not to state exactly what is required but just state what the end reslut must be is to be flexible.
Most engineers will comply with an AD as it is written but when appropriate they had a statement something like "or alternative means of compliance acceptable to..........."
Although rarely used this allows an engineer who has a better and/or cheaper idea for compliance can do so as long as it is acceptable.
Some time ago I received an AD that required a 300 quid part plus 5 manhours to replace it, it was intended to prevent failure due to chaffing. I managed to show that minor re-routing and application of 2 quids worth of plastic trim would effectively make chaffing impossible thereby meeting the alternative clause. We saved over 300 quid for the owner. The manufacturer wanted to hear nothing of it but the CAA accepted the alternative compliance.
With reference to the seat repair, I think it is quite obvious from this thread as to why it needs a piece of paper approving it. The statement "nothing to do with powerplant, structure or controls" displays that the author has missed the dangers of a seat failure.
The piece of paper would show that the correct investigation and calculations have been performed to demonstrate strength and reliability.
XX621 please get it checked out.