This has been discussed a number of times on this forum so I'd recommend reviewing some of the older posts and discussion threads. There is not universal agreement on this, but in my opinion, because a screening colonoscopy is a preventive service, a consultation or other E&M office visit code that is for reporting the evaluation and management of a problem is not appropriate. All of those codes require a chief complaint, and there is none for a screening and so by extension, as you point out, no way to properly score diagnosis points in order to support an E&M level.
Unless there is a symptom, problem, or other comorbidity that requires physician evaluation prior to the procedure, the medical necessity of this visit is not supported. Under global surgery rules, the initial evaluation and decision to perform a colonoscopy is a component of the global surgical payment and should not be billed separately. Many GI practices screen patients by phone interview prior to the visit and do not schedule a separate face-to-face visit with the physician unless that interview identifies a problem or concern that needs to be addressed by the physician in advance. A separate visit done solely for the convenience of the patient or provider is, by most definitions, not medically necessary.
Some payers do offer a preventive benefit for a patient to have a consultation with the physician prior to a colonoscopy, in which case that service may be billed with HCPCS code S0285, or with whatever code that payer designates in their policies should be submitted.