CMS has a tutorial on ABNs:
https://www.cms.gov/Outreach-and-Ed.../ABN-Tutorial/formCMSR131tutorial111915f.html
Think about what your question is. If you consider it closely I think you will see you answered it yourself. The A in ABN stands for ADVANCE. It means you can't sumbit a claim to see if it's denied and then decide later to issue an ABN, it has to be done before (in advance).
The provider can't balance bill the beneficiary just because the provider doesn't like the reimbursement amount from Medicare.
"50 - Advance Beneficiary Notice of Non-coverage (ABN) (Rev. 10862; Issued: 07-14-21; Effective: 10-14-21; Implementation: 10-14-21) A. General Statutory Authority - Applicability to Limitation on Liability (LOL) Section 1879 of the Act (where the LOL provisions are located) requires a healthcare provider or supplier (i.e. notifier) to notify a beneficiary
in advance of furnishing an item or service when s/he believes that items or services will likely be denied by Medicare for any of the reasons specified in the statutory provision in order to shift financial liability to the beneficiary for the denial. For example, advance notice is required if the item or service may be denied as not reasonable and necessary under §1862(a)(1) of the Act or because the item or service constitutes custodial care under §1862(a)(9) of the Act. Notice (e.g., the ABN) is a way for healthcare providers or suppliers to establish beneficiary knowledge of non-coverage and therefore, shift financial liability for these items or services if Medicare denies the claim"
Regarding unlisted CPT: Your MAC has instructions on properly submitting claims with unlisted CPT. It requires PWK and explanation with documentation. Here is a Noridian example:
https://med.noridianmedicare.com/we...rs-solutions/unlisted-procedure-and-noc-codes