See the fish in the pond and the penguins face in the pond? Does the piping gel not eventually turn the characters into mush?
I, too, made a castle cake with a piping gel moat. I placed gum paste water lilies on it and didn't have any trouble. The gum paste was completely dry before I used them and they only sat on the surface, not embedded in the gel. I see, however, that you are referring to fondant melting, not gum paste. All I can tell you is that the fondant bottom of the moat seemed unaffected by the gel on top. As it was the fondant covered cake board, it was fully dry before I spread the moat on top.
<a href="http://cakecentral.com/g/i/2139120/emma-couldnt-decide-which-disney-princess-was-her-favorite-so-all-were-invited-to-the-party-at-the-pink-castle-since-this-is-a-friendly-castle-there-is-no-drawbridge-over-the-piping-gel-moat-rather-a-welcome-bridge-the-cake-is-wasc-with/#"><img src="http://cakecentral.com/image/id/268998/width/400/flags/LL"></a>
I hope this helps you.
my mm fondant is dry on the cake i think I will send the piping gel with them to put on when they get there . just in case i read someone said as it dried it became cracked I have chocolate buttercream and cake under the blue marshmellow fondant so it would realy show if it cracked. my gumpaste fish and boat are mixed with wilton fondant. so I just hope they dont melt and slup over once on the piping gel.
cazza1, I remember your talking about this cake. I just loved it, but I can see, (figuratively speaking-- for it isn't visible to me), why you were upset. I guess the secret is the fact that your gel is much thicker than mine was and your packages are embedded in it. And I guess sea water isn't really recommended for "color safe" wrapping paper. I love the story this cake tells!