No they couldn't, not without causing a problem for their own Administration... because it is against the law.
Federal law makes it a crime for an IRS agent (or any other federal employee) willfully to disclose unauthorized tax returns. If an agent did indeed willfully release President Trump’s unauthorized tax return, he or she could be subject to five years in prison and a fine of up to $5,000. Upon conviction, federal law provides that he or she would also lose his or her job.
So. No, they really couldn't. The government acting in such a way by order of a President would absolutely be a "high crime".