You are prohibited from disclosing:
* a grand jury subpoena or
* a national security letter.
As your instincts suggest, a levy is confidential, but there isn't anything that says you cannot disclose it in a SAR. It might be very helpful to law enforcement; e.g. IRS CID if you do so.
In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.