I never saw your other thread (
http://www.canreef.com/phpBB2/viewtopic.php?t=4056) until now, so I wasn't sure what you had meant. Now I can kind of visualize this now, whereas before I didn't at all understand what you were trying to describe about the gap.
Honestly, what I'd do, drain the tank maybe a couple inches (yes this will mean you are disconnecting from the sump, temporarily), and just go with silicone. I don't see why this won't work. It's not under the waterline so there shouldn't be any issues of toxicity. Maybe use a fan if the fumes/smell is a concern, but seeing as it's only a small amount I don't see why this should pose a problem.
I would go with silicone, over anything else. That other stuff ... who knows how runny it is and what would happen if you spilled some (maybe harmless, but why find out the hard way that it isn't, if it isn't?). Maybe ... put psome masking tape under the gap before applying the silicone, and then for sure none should likely spill into the water. For sure I don't think there's any reason for it to be any harder on yourself than this ... this isn't that bad a thing to do...
Dropping the water level isn't that big of a deal. Think of it as a prolonged water change.
When I first suggested the epoxy I was somehow thinking this was under the waterline.
BUT ... I still stand by my earlier assessment that your water level is too high ... if I understand this correctly you are saying that there is only 1/8" bewteen your water level and your bracing (presumably euro bracing btw). I don't think that's really enough. Just my opinion of course... Smack one to the guy who built your overflows, that's a very large oversight in my opinion... it would be one thing if you had a tremendous amount of flow from your sump return meaning your water level was >1" above your overflows... but you don't have that much if I recall correctly.