Get a kneeling chair, wear your guitar on the strap as if you were standing, but no weight on your feet. When I started studying with a professor from NE Conservatory, my FIRST homework assignment was to get a kneeling chair.
It feels different, and more important - I use different left hand approaches:
Thumb behind the neck -
Left arm has absolute freedom but keeping it angled up for too long (more than 1/2 hour) is hard for me
I grab the neck when I play standing (did anyone mentioned Jimmi Page?) and angle my wrist - that's completely different hand position, twice less stratch but it's comfortable it's own way.
I've found that keeping the left arm/wrist/fingers very loose is the only way for me to use that approach
Sometimes I switch to classical lefthand position ( rising the neck up to 45 degree helps - take a look at Chuck Schuldiner)
I switch standing/sitting positions during practice every 1/2 hour or so..
i brought my combo up to my bedroom this week, and have beem practicing while laying down. propped up with four pillows, natch. if i can find a couterweight/sling rig to eliminate static gravity pull on my arms, i will have achieved the ultimate in relaxing, untransferable-to-live guitar practice!