Can soldiers in civilian clothes salute a fallen comrade memorial?

YES.
Saluting is a sign of respect. Be proud that you are able to do it.

I think it would be foolish and unpatriotic for anyone to ever say NO to a veteran or un-uniformed soldier to show their respect to a tribute to our fallen comrade. Giving a simple hand Salute is OUR way of silently saying thanks and we come and leave in peace My Brothers and Sisters.

That man's salute may have been his only opportunity to honor or say farewell to fallen or injured soldiers from his unit, loved ones or an emotional release of PTSD from a horrific, socially devicive war.

Yes, Saluting the fallen is out of respect for the ultimate sacrifice. We veterans and those still serving respect those who gave all. Salute them and be proud to do so.

So who, exactly, is going to stop a veteran from saluting? As someone mentioned (I think 6 years ago on this post) the policy/law was a waste of time. I have no qualms about saluting in civilian clothes for the National Anthem, or for fallen service members or even when I greet someone who served along with me.

I do and I have in the past, for me it's a sign of respect, not sure what's the issue with that ??

Active duty, Reserve Component and Veterans are entitled and encouraged to render the hand salute when appropriate. Says something about how few of us there really are when you go to a ball game or monument, and see that very few of us do that. You show respect and honor and brotherhood regardless of what you do; but, you add gratitude for having the opportunity to serve shows in the salute.

Do what brings honor to the service and your uniform. In the absence of specific regulation, you need to use your own judgement. I doubt any civilians know the exact regulations regarding salutes, and would likely find it inspiring and or heroic-looking. If you want to salute, then salute. It is your gesture of respect. If some SgtMaj or 1stSgt with something to prove gets mad at you, treat him or her with the professional courtesy and obedience to proper orders they are due. If you are looking nasty or are in a situation that would reflect poorly on the military, however, don't salute or make any military reference. The regulations are clear on that. Essentially, pay attention to optics.

I do it all the time, feel like its my own personal preference.

The act of saluting is more important than clothing wear.