unusual toilet seat

I know, I know, sealing the bathtub overflow drain is not a wise idea under most circumstances. However, I am remodeling our bathroom and there is a shower drain on the floor next to the tub, so any overflow problems would be caught by the shower drain. It's basically being remodeled into a Japanese Bath where the tub sits directly next to the shower and overflow of bathwater is common and expected. We have a Kohler Greek soaking tub. It's pretty deep, about 22" of bathing depth; however, the overflow drain reduces the bathing well to about 18". THerefore, we want to seal the overflow drain so the water can reach the top of the tub. What is the best way to seal the overflow drain? They apparently don't make a faceplate that seals the overflow drain. All faceplates I've seen have a space underneath to allow for water overflow. I've tried to seal that area with silicone, but it looks terrible. I used a piece of PVC shower liner and silicone in an attempt to seal the overflow drain prior to attaching the faceplate, but that didn't really work out either. The PVC shower liner kind of bent a little when I screwed in the faceplate, and I don't think I got a good seal. I'm a little concerned some water will therefore get trapped behind the faceplate and create some rusting problems. I was thinking of using Car Bondo to seal the overflow drain, but then I might have a problem screwing in the faceplate... What do you guys think? Whatever I do, I would like to do something that looks a little better than the crappy silicone job I did.

1. Purchased a new water softener ($5k) 3-4 years ago. 2. Last week build-up from pipes and silica beads were coming out. 3. Ran the water until it was clear. 4. Was OK for a day. 5. The next day, hot and cold water was clear but the hot water had a very chemical odor to it (like WD-40). 3. System is timed to flush out for 20 minutes every night 4. Last night we purged the flush. 5. This morning slight build-up, mostly the silica beads came out again and some aerators in the kitchen and bathroom are clogged again. Tho, the amount was substantially less than the first time it happened. Is this a pipe problem or a water softener problem?