Not surprised no one has posted a answer;

Interesting question as I secured a Uberti Low Wall in 32-40 (Scheutzen styled stock) (12 twist) some time ago. 
I haven't done much with it so far and had planned to start out slow after I secured 'Quick Load' software, which I still need to do.

As such I will be following this thread closely. 

Consequently I have no answer either, only a thought on how to dig deeper.

First you have to find someone with ballistic software like quickload or (You need to Login or Register to view media files and links)

Then, if you can find it determine the max yield and compressive strength  of a Low Wall action. 
if your Low Wall is an original, I'd be more concerned with the metallurgy then on a reproduction Low wall. 

There was a graph floating around some time back which referenced Low Wall cartridge compatibility which you have probably seen. I will try to include the image of same below.

My CPA 32-20 shoots a 207 grain bullet without any compromise of the brass itself.  

It's not really weight, it's basically, how long the bullet is.

You can shoot a .83 long bullet, in a 30 cal 20 twist

I don't think a 150 gr bullet would be to much for a 32/20, in a LW.

I built a a 32/20 with a 11 twist, for my wife, on a 44 action and shot 160 & 184 gr BSed bullets at 1300. 

In my own LW, I cambered it for 357 Max (I call it a 38XL) and shot 208 gr bullets. PB at 1400 and GC (35875 & 358315) at as high as 1800+. It's still going strong.

Here is a Lyman/Ideal that could be copied at Accurate, that should work well BSed. Or, you can use the 130 gr 311410, if you can find one.

I'm not saying that anyone needs a 150 gr bullet. The question was what would stabilize in a 20 twist.

So, if you like you can have options of up to a ~150/.83 long bullet. Although, BSed,  I think it would have a accuracy advantage.

Regarding CPA 32/20's they are all actually a full 32 cal and don't apply here.