Can soldiers in civilian clothes salute a fallen comrade memorial?

Yes

Absolutely. My veterans motorcycle group does it routinely when we do flag lines. I do it in and out of uniform. I rendered the hand salute to the urn that held the ashes of an army buddy who took his own life. I will do it again this weekend at drill for one of the guys in my unit passed away in his sleep last month just before drill.

My father is buried in a national Cemetery. When I go to visit him, I ALWAYS toss him a highball. If you don't like it, if you think it improper, that's fine.
Just don't watch.

It is a very personal thing. Any veteran may have a deep connection with persons being honored, just as non-veterans have when placing stones or coins on on headstones. Saluting is second nature to veterans....

To respect soldier

Salute! Veterans can now salute while in civilian attires. Same goes for retirees and active duty personnel.
Like everything else, things change, and sometimes for the better.
I salute for the national anthem and at funerals. I had done this long before it came into law or a regulation.

As You Stated SPC ???....
"So my question is actually coming from me as I've felt compelled to salute as well, but have refrained as I'm not sure what is correct"
SCREW A Bunch Of "CORRECT" Crap:
We're All Brothers Deserving Of Giving Each Other, Both Our Respect And Love
SNAP That Right Hand To Your Forehead Where You Wish, And Where It Belongs

Hey. Honestly, I never thought anything about it. I did it for 27+ years on active duty. I conducted about 40 military funerals, so many times conducting reveille, retreat, 3 tours, I did it out of respect and it was custom n courtesies. Plus as a Veteran it is our responsibility to pay honor to our flag and fallen comrades. No one can tell me otherwise. especially some ass clown politician.

Going to sporting events, I was never in uniform and always saluted when they presented the colors and the National Anthem. No one around me said anything against. So unless your one of those hate n discontent jackoffs, then.....you're on your own. I've got no time for you.

If someone tried to stop me they had better "Check their self, before they wreck their self "

I salute the flag when the anthem plays and when we recite the pledge. Nobody has ever said that I couldn't or even questioned me. I think it would go to our first amendment protections but even so, we've earned the right.