I haven’t had this problem either; I think it just depends on what whistles you’re using. From what I understand some whistles have very narrow windways making this problem more likely, but I guess I’ve never played one such.
I can’t help but picture these soap applications leading to one blowing bubbles out of one’s whistle. I’m sure it doesn’t happen, but a humorous mental picture at any rate.
I had that image too. And couldn’t help but think it would be a useful thing to have at West Ham away matches. (Brits will understand.)
Seems there is a toothpaste and a soap faction here… And maybe a scotch faction. Well, I’ve tried the toothpaste treatment and it helped quite well so far. Condensation problems have decreased at least. A big Thank You for your help 
Maybe scotch is the most effective method. Depending on the amount, you probably won’t realize any condensation problems anymore…
Thanks again - FG