OK, so I just did the boiling some beer thing.
I boiled 250ml of beer (belgian blonde ale, about 7 ebc, 6% abv) down to nearly nothing - it probably got down to 15-20ml before it went syrupy in any way. The smell coming off the stuff (after it stopped smelling frankly nasty at any rate) was all about bread crust and toast.. no caramel.
So I ended up with this
I added back enough boiling water to make the volume back up to 250ml, making sure I scraped down the sides of the pot and got back as much of what went in as possible. This is what came out. A glass of the original beer on the left - the reconstituted syrup on the right
So - got a bit of colour development then.
And this is what happens when you add some of this reconstituted syrup back into your beer. I added 10ml to 90ml of the original beer to simulate what would happen if you did this to 10% of your keg ( a couple of liters)
original left, altered right
Added a little colour. How's it taste??
Well, it didn't add any caramel - it added maltiness. So the difference between pale and munich. It was toasted breadcrust all the way. thats what it smelled like, thats what it tasted like.
A 20% solution added a very small amount of what you might call caramel/toffee - but in that burned and slightly sour way you get when your toffee goes wrong. Bit of extra body, definite strong maltiness/kilned character and an edge of astringent bitter/sour.
So I tasted the tincture itself... not all that pleasant. I have tasted the same thing when making malt "tea" to learn about malt flavour - got it from amber malt, got it from melanoidin malt. Bitter, puckering, sour acid structure like you get in an espresso. Also there were significant floaties and haze -- It would take a bright beer down a peg or two in the clarity stakes to add 10% of this stuff.
Conclusion -- you could do this to add colour and maltiness to a beer. A little structure and mouthfeel perhaps ... but caramel??? Not so much. Worth it? Only if you were desperate to save a truly bland beer.
TB