I find this quite an interesting idea - more interesting given the fact that it is being floated by someone with extensive experience using silicon in a non trivial way.
Concerns for me would be
1 - OK silicon is strong. Is it going to stay that way after it gets heated to 100 for a couple of hours, cooled down again and repeat..... presumably dozens of times. I have no doubt it would hold the first time - will it hold the 20th or 50th time? You dont want to find out the answer is no just as your favourite kitten walks past the brew pot. I've never heard of silicon being used in this sort of application before... maybe it is and its just something I've never heard of, but it would be worth checking out before you decide to go ahead. If its not used this way, there's probably a reason.
2 - My shower. In my shower there is silicon sealant stopping the water I rinse my goolies with, from falling on the downstairs neighbor's heads. It doesn't leak - but there is an impressive array of unscrubable mold growing between it and the tiles. Unscrubable things make brewers sad. No idea if that would happen with the way you intend to bond the surfaces - but you're the pro and its a question I'd be asking of you if this were my idea
3 - Internal shame. You spend $40 on 2 woolies pots, an hour or two of your time cutting them up and sticking them back together again and you will have.... an ugly, cheap arsed, stuck together work around pot. Sure it might work perfectly and if ghetto cheaping out is your thing - it will probably bring a smile to your face every time you use it. BUT - if you're like most homebrewers I've ever met - what'll actually happen is that you're going to spend the next year or so looking at that thing and dying inside a tiny bit whenever you see it. Eventually you'll cough up the cash to buy the pot that deep inside you always knew you should have bought in the first place, relegate the silicon wonder to the darkest back coner of the shed and wish you'd saved yourself the $40 and the thinning of your self esteem.
I sort of want you to do it just to prove it works - hell, I remember asking on this forum once why people were so fixated on tig/mig welding of fittings for brew vessels when a nice silver solder job could be done at home by anyone with a bernzomatic torch. People howled and told me how the solder would melt on my burner and how it would never be strong enough. I considered offering to solder two bits of brass together and paying $1000 to anyone who managed to get them apart again with anything short of a pair of bolt cutters and another torch.....
I dont think its that good an idea and I dont think you should actually do it ... but I do like the lateral thinking and I do think people need to open their minds up a bit on occasion (except me of course)
TB