A comparator placed next to a cauldron can be used to know how full a cauldron is (outputs power 0, 1, 2, or 3). Would it be possible to modify this behavior so that the output of the comparator is proportional to the fermenting time?

Example: x minutes of fermenting => power: x (assuming that no recipes require fermentation process longer than 15 minutes)

Moreover, I noticed that the content of a broken barrel leaks out. This gave me the following idea to automatically collect the brews after X days:

1. Place a sticky piston next to a part of the barrel.
2. Connect the piston to a day counting machine. The principle is that every day a block falls in a chest (uses a daylight sensor).
3. When the time comes to remove the brews from the barrel, the sticky piston retracts. The content of the barrel comes out and is collected by a hopper.

So far everything works fine. However, the barrel is not recognized as such once it is reconstructed (i.e. when the sticky piston is powered). The only solution is to remove the Barrel sign and replace it.

I believe that the validity of a barrel is determined once when a Barrel sign is added to it. An alternative that would solve the automation issue would be to keep track of the UIDs of the Barrel sign and check at regular interval if they are part of a valid barrel.

Because the proposed solution would require more computational resources (checking barrel validity at regular interval vs. only at creation time), an option should be added to the config file.

Thinking about the next step, is its possible to fill a brewing stand with hoppers?

Yes

The apparent problems with redstone technic is that it does not update/work when a chunk is not loaded.

I would have to test with the Brewery cauldron and barrel but redstone machines work continuously in the spawn chunk.

I really like this idea! I like the automation of stuff an that seems like some good ideas for it.
Having the comparator show the time a cauldron spent fermenting would be awesome! Though it would need intercepting the already performance heavy redstone mechanics. But done right it could work. This would also resolve the issue of having to run to each cauldron and checking its time with a watch. A line of Redstone can then show the time nicely.

The next step would be using hoppers with empty bottles in them to automatically fill from the cauldron.
Thinking about the next step, is its possible to fill a brewing stand with hoppers?

Also needed: Hopper support for barrels! Filling and emptying them.
That could be easier than breaking the barrel to make the brews flow into a hopper, because keeping the broken barrel and checking it for rebuilding will be performance heavy. Maybe still worth implementing, because accidentally breaking a barrel and rebuilding it would then also not require a sign again...

The apparent problems with redstone technic is that it does not update/work when a chunk is not loaded, while barrels and cauldrons do continue to ferment/age.