I carry a halcyon 80# closed circut bag and spool solely for lift bag use in one of my drysuit thigh pockets.
And as scubanarc described, I keep a 100# carter open circut bag secured to the bottom of my BP with sugical tubing loops.
I applaude you for wanting to carry a lift bag on all dives, excellent descision. Remember though, you need enough line (spool, reel, etc..) to get it to the surface from any depth you might *theoreticaly* be at... and don't take the manuf. specs as truth when it comes to how much line is on that reel... measure it... if for nothing else more than peace of mind that you KNOW you have enough line at depth.
As for where to put it on a BC... well, I have seen alot of people just clip lift bags to a shoulder strap D-ring, but... well... it's cumbersome... and... well... it's in the way... and... aw, you get the picture, try and find another way if you can... folding it into a pocket has worked nicely for me with the CC bag. I don't really have any experience with a subsalve 50# though, I don't know if that's a feasable solution... I've only seen the bigger ones..