Yes! And hot of the press legal precedent has been set as of today which would create a means of arguing such a case before a court.
Three hours ago when I first read your comment I thought No! It would not be a free speech issue.
But then I read this ruling which came down today which tracks in parallel with this issue. In almost every line of this 74 page ruling I can see how the application of law to the Twitter First Amendment issue applies to this issue. The whole time I was reading the ruling I kept seeing how this is parallel with that.
Here's the ruling:
https://www.lawfareblog.com/document-tr ... strict-new The key elements which I think would be needed for standing is a player(s) who wants to speak, and a fan(s) who wants to observe that speech. One would have to show that the anthem is government speech and that within the stadium there is a public forum for that government speech to be commented upon. All of that in light of todays ruling for Knights First Amendment Institute v. Trump seems quite straight forward.
There's one more essential element needed to make a case, this would be difficult, one would have successfully argue that Trump in publicly fussing at the owners of the NFL to penalize players who kneel caused the NFL owners to act.

From the Cornell Law School citing U.S. law, 36 U.S. Code 301, on the National Anthem:
Quote:
(a)Designation.—
The composition consisting of the words and music known as the Star-Spangled Banner is the national anthem.
(b)Conduct During Playing.—During a rendition of the national anthem—
(1) when the flag is displayed—
(A) individuals in uniform
should give the military salute at the first note of the anthem and maintain that position until the last note;
(B) members of the Armed Forces and veterans who are present but not in uniform
may render the military salute in the manner provided for individuals in uniform; and
(C) all other persons present
should face the flag and stand at attention with their right hand over the heart, and men not in uniform, if applicable,
should remove their headdress with their right hand and hold it at the left shoulder, the hand being over the heart; and
(2) when the flag is not displayed, all present
should face toward the music and act in the same manner they would if the flag were displayed.
Cornell Law SchoolNote, the law says "should" and "may" indicating it is not mandatory but what a person is encouraged to do. If the law were to be enforced then "should" and "may" would be "shall" or "will" to indicate mandatory action.