Belgarath, you’re right that stating a plan provision in a way that doesn’t follow an adoption-agreement form’s instructions loses reliance on the IRS’s preapproval.
Revenue Procedure 2017-41, 2017-29 I.R.B. (July 17, 2017) at its § 6.03(17) states: “Opinion letters will not be issued for . . . Plans that include blanks or fill-in provisions for the employer to complete, unless the provisions have parameters that preclude the employer from completing the provisions in a manner that could violate the [tax-]qualification requirements[.]”
No comment about whether an allocation fits the preapproved document you mention.