Can dielectric grease be used in coax connections?

I have never used it and am not familiar with the ingredients of that product but generally where RF is concerned you do not want to add something that was not already called for in its application. You said the connectors are water-resistant (water proof is hard but not impossible).

Adding anything at all will introduce performance changes and not for the better (i.e., its not going to boost the signal without active components), it will likely add some filter/tuning impacts but not something you can really control and again, not for the better.

Why add it? Is there a different problem you are trying to solve? If you are aiming for an anti-sieze application, it could work, just read the product label to ensure it doesn't react with the coating or metals used on the connectors and barrels you are using. Use it sparingly on the outer female threads only. Pay mind to avoid the center conductor and the insulator between the threads and the center. Also avoid the very first thread to avoid sandwiching it into the insulator or center connector when tightening it.

Consider STUF dielectric filler, Made for coax, Saw a new video on it : https://www.youtube.com/watch?v=2l0PqZr8-_o

There's footage of a coax male end packed with the filler which is white in color. The claim in the video is

Prevents moisture infiltration by occupation

The packaging reads

Dielectric water proofing filler: for coaxial connections. With Teflon.

The company website links to www.crossdevices.com and coax does seem to be a specific and focused application of theirs.

STUF is actually an acronym

Silica Teflon Unionizing Filler

They claim to match the dielectric constant and impedance matching to the polyethelyne insulating foam surrounding the conductor.