How much can we lower the tuning on a classical guitar?

Aquila has a "low A" string set for baritone ADGCEA tuning, and even a "low E" set for EADGBE - one octave lower than standard (same as Bass VI tuning). Both are intended for normal scale classical guitars.

All strings are wound in both sets. I think the low-A set sounds better, which shouldn't be surprising given the instrument being quite small for such a low range.

Also the low-E tuning asks for a more bass type of right hand technique to sound good.

I don't think I've seen baritone gone past B on a Fender-scale electric. I think you'll have problems with setup and intonation, but low tension, not high, will be the crux of your frustrations.

For strings, take a standard (heavy) string set and toss the high E string and ... I honestly don't know where to go for a thicker nylon/gut string, but you should be able to find singles at a string outlet like Just Strings or maybe your local shop. I'd say .053 or so would get you in the ballpark.

(Ironically, early rock guitarists would get standard (.012s) sets, toss the low E, get a thin banjo string and that's how we got the .009s we do often use these days.)

As a former fretless guitar owner and current steel guitar and slide guitar player, sustain is the best friend I have when working without frets. My suggestion for fretless guitar is an electric with a humbucker and maybe a Sustainer or the like. Either way, guitarists come up believing that intonation is something you set with a screwdriver, not something your ears and fingers work out. It took a long time to get to my current state of mental intonation, and unless you come from a strings (violin, cello, etc) background, that will be your learning curve as well.

Part of me thinks "look into Ouds; they're built for this sort of thing", but I hope you find the sounds you want in this journey.

Looks like you’re going for a classical guitar version of a baritone guitar, which are electric steel strings, have a longer scale length and are usually tuned a 4th or sometimes a 5th lower than a regular guitar.

It seems like you can take a regular set of strings but offset them by 1 position so the 2nd string would be where the 1st goes, etc. They would be tuned about a step lower than they normally would, A instead of B, E instead of G, C instead of D, etc. so maybe a high tension set might work better.

The problem is finding a string to serve as your low A. Maybe from a Mexican Guitarrón or Bajo Sexto? I’m not sure if the latter uses nylon or steel strings. The other issue is because of the short scale length the lowest string might not have a very good sound.

Like you mentioned, I would also be concerned about stressing the instrument so if you have a less expensive guitar you wouldn’t mind experimenting with that would be the way to go.