Requirements for contractors
Sounds like you have a lot of good stuff in place already and looking to fill in the gaps.
So as these are all variances particular to your organisation and they are over and above those required in regulation / standards, then I'd start with the name of the Regulation/Standard and have the engineers list the variances required against it. This will build you a register of company specific requirements per standard. If a standard is updated or an in incident occurs then you can review/update the register of variances accordingly.
The Management of Change procedure usually starts when a project makes it off the wish list, (so all stakeholders have an initial chance for input), so you could include an ENG/HS check that a list of variances per standard has been given to the contractors or if the job refers to a standard not on the register, that a review of the standard and a listing of variances is added to the register as a prerequisite of talking to contractors.
Do let us know what feedback you got from the engineers and other managers.