Generally [with the "long hose" setup], in an OOA the donor uses his/her right hand to remove the primary second stage from their mouth and offer it to the recipient...
(And, as an aside, the donor actively gives the second stage to the OOA diver, rather than letting the OOA diver find and retrieve it, so having a yellow purge cover on the primary, or a yellow hose, is unnecessary - nothing wrong with it, just not required.)
After reading this, I went back and re-read my post and I want to clarify a couple of things I probably could have expressed better.
On the yellow purge cover on my primary: I said "then they will know what to grab," but then Colliam7 rightly pointed out that typically the long-hose donor will actively
give the reg to the OOA diver. And I don't disagree with that. I'm guilty of having a scenario in my head that I didn't fully express, which is that most of my diving is from boats, and so far just about all of the other divers (whom I have no previous connection with) dive the "usual" setup where they come and look for the reg if they need it. And although we drill, my usual buddy also uses that "usual" set-up, and his alternate is yellow.
So between those considerations, and the fact that I already had one black and one yellow purge cover, I decided to keep one yellow (at least so far) and simply move it to my primary reg. That way if someone in proximity (who is not my buddy) needs air, and does come up to "grab," (maybe unexpectedly), they will have a target. Too, it seemed to help with DM's who were unfamiliar with my set-up. That's not to say that I might not change it in future (I could order a black purge cover).
The second thing is that I said something like "there are plusses and minuses to a 5' hose in open water." Re-reading, I could see that it might sound like I meant "as compared to the "usual" setup. But what I was thinking when I wrote it was "as compared to just going with a 7' hose."
The main minus, for me, is that the hose can tend to "float up" in back (behind my head) and give me a sort of "bucket bail" effect over my head. That's annoying. I don't think a 7' hose would do that, because it would be anchored on my right hip, and thus come up on a different angle (I have not tried one though).
On the "plus" side, as I mentioned, I typically dive from boats, with people used to the "usual" setup,
and I doff my gear in the water and hand it up to the DM. For that I don't mind at all that there is not two more feet of hose. I can clip off my primary and the dangly loop is more minimal. 7' would be "more snake" in that case. Also, the 5' hose is plenty long for me (5'5", short torso), and seems positively roomy on air shares compared to my usual buddy's 30-something-inch(?) hose.
If you are 6'6" though, 5' would very likely be too short for you, as Colliam7 mentioned (although it might depend on your build as well). My usual buddy is only 5'10" - but barrel-chested - and we switched regs one day for him to try the long(er) hose. It was ridiculous on him, and put the primary off to the side of his mouth like a Popeye pipe. Too short!
I can't speak to any of the alternate routings, but I can say that the "standard" routing of the long hose works beautifully when it comes time to donate. Before you try it it seems like it will kind of be "stuck" behind your head - or like you will have to do gymnastics to donate - but in reality it comes out in one smooth, easy motion, with just a very slight duck of the head. I do it on the boat right after I put my rig on, just to be sure all is free and operational, and it's just trivial there or in the water. At least that's how it works for me.
I clip my SPG off at my left hip D-ring, and although I do un-clip it to view it, I don't tend to "have to do flips" to view it. I just reach down and un-clip it (motor memory/feel), and then bring it up to where I can view it, before re-clipping it (motor memory/feel). So I move the SPG but not my torso. That's not to say you might not prefer a different way, but just to say that I don't have to change my body position to view it. I do have to reach down to unclip it and re-clip it though, as I cannot see it without moving it.