To what extent can I deviate from the endorsement guidance given in AC 61-65?
Advisory Circulars by definition, are advisory and not regulatory in nature (Chapter 1, paragraph 3 of FAA Order 1320.46C). Therefore, AC 61-65 provides suggested language that will ensure compliance with the FARs.  You are free to word the endorsements differently, but if the examiner takes issue with your endorsement, the student may not have any recourse but to have the entries redone (by you, or another CFI).

Since you say your student is ready (I am assuming for their Private Pilot check ride), why not go ahead and endorse them right now? FAR 61.109(a)(4) requires the student to receive 3 hours of flight training within the two months prior to the test, this would cause your endorsement to automatically expire if they don't test (or receive additional training) within that window.

Alternatively, I would suggest contacting the local Flight Standards District Office and get guidance from them directly.