I'm not the best in term of defining policies but I would rather formulate it that way:
Title: Key Users and Business Process Owner Role Assignment
Content: In lieu and place of the Lead Link, the Key Users role is assigned to anyone filling the Business Process Owner role. Further, the partners filling only the Key Users role won't be considered as core circle members but only as circle member and thus, will be excluded from meetings. The Business Process Owner role-fillers will carry the purpose and accountabilities of a Rep Link in the context of the Key Users role and for any circle member filling that role.
I don't know if that's perfectly aligned with the constitution nor with the english language but I'm sure you get the sense of it.
You don't need to specify “and that, further, such individuals shall represent both the Business Process Owner role and the Key Users role in all circle meetings.” because unless you specify otherwise, it is by default but you do need to precise that they act as a Rep Link, processing tensions for others.
Hope that helps!