I play with my right arm totally free and once I got used to it, I feel regular posture hinders my RH.
There is a famous player who also felt it nice and then he decided to develop a whole new posture and technique because of this. His name is Paul Galbraith.
I studied for a couple years with Everton Gloeden, a close friend of Paul, they played together in the Brazilian Guitar Quartet for several years, and Everton also decided to change his technique following Paul's path.
I followed my teacher and I've learnt to adopt the posture without the need of the cello-like end pin. It is possible to achieve the posture with an Ergoplay, Muratta or similar at the back of the guitar (search youtube for BGQ videos you can see my teacher in action).
We also play with the guitar more in a diagonal position, to achieve more RH angle, instead of Paul's frontal attack. I play it even more diagonal than my teacher. I think I play in a posture similar to some players like Adriano del Sal or Gabriel Bianco, I just twist the guitar up a little bit, so I can free up the right arm. And with the twist, I think I can get more angle than they do, but I've been playing with less angle lately...
Another important tip is that we use a suction cup on the side of the guitar, tied to the pants or around the leg, to prevent guitar from slipping (so you don't need to hold the guitar with right arm).
With the guitar more up, diagonal, or cello-like, right elbow is low, so there is no problem holding it. It tires a bit at the beginning, but after around 3 months I got used to it and it is fine to play several hours with no problem. Actually I think this final posture is ergonomically better than the traditional one, where in the latter we tend to twist the spine, move the shoulders to the front or have trouble reaching higher frets. Big problem to me, in the new posture, is keeping good RH angle and tone. Paul's tone is not the best to me, I think it is a bit brighty and thin to my taste.
Finally, I think it is really tricky to find out good posture with this, specially if you consider there are almost no references. I don't think I would be able to learn it that fast without my teacher guidance. My teacher really likes it and he admires Paul a lot. He says freeing up right arm is the next step after Tarrega's removing pinkie from soundboard... But he never forces his students with this posture. He keeps teaching them regular posture unless they ask to change. I know only one guy besides myself who also really adopted this new posture.
Another comment, a problem with the strap posture above is that full the body contact with the back of the guitar may prevent full vibration and full sound of the guitar. Paul used to hold the guitar with his legs and toward his body when he began with free right arm thing. After an observation by Sergio Abreu that it was hindering guitar vibration, Paul ended up changing to the cello-like posture with the pin. A lot of people think Paul reached that posture imitating from cello. But it was not like that. It was a whole process of freeing up right arm until he finally reached the cello-like style.
People also think the 8-strings Brahms guitar has to do with the cello-style. But they are also not connected. Paul just ended up developing both things more or less at the same time. My teacher plays regular 6-string as I do, with this posture. So did Paul in the beginning. My teacher ended up playing the 8-strings Brahms in the quartet, to extend possibilities of the 4-guitar arrangements.
So it is a pity a lot of players don't try Paul's ideas because it seems complicated with a cello-pin and 8-strings guitars. If they just knew it is possible to try it with a regular guitar and regular accessories...