It's been a big wkend for me so my brain is going a lil slow. But this is where it would begin.......
"why does only loperamide cause excess urination?"
all other opiates tend to make you pee less (and poop less), yet there are many, many reports of loperamide causing excessive urination and dehydration. yes it definitely constipates, but why the excess urination and dehydration?
it does wonders for withdrawal its unbelievable, but i myself find myself going to the bathroom every 10 minutes sometimes, and no matter how much water i drink my eyes/mouth are dry and i just cant retain water! what the fuck?!
A little big of anatomy/physiology (just in case

) - in your brain your nervous system can be broke down into groups and then subgroups. Two of these subgroups are called the Sympathetic Nervous System - this generally deals with what's called the THREE F's - fight, flight and, err, "making love".
One of the other ones, which you're interested in here is the Parasympathetic Nervous System - this deals with "rest and digest". So, when you're trying to sleep at night, it's you're PNS kicking in making it happens. Now there's aren't just opioid receptors in the brain, they're a few places in the body, one of them being in the Gut/GI track!
So as you said, most opiates cause constipation since what's happening is those opioid receptors in the gut are being "fire", which when those kinda receptors are in the cut, and they get activated, that's the end result!
The only two other possibilities I could think of (especially since I don't work in addiction) is that firstly (and especially if this has always happened to you) from human to human there exists a thing called "bio-variability" so a drug might work one way for you, and yet another way for another.
Or lastly, addiction affects the body (obviously depending on the drug and that too but) a many, many ways other then you take a drug and ya get ya high! Again, also depends on the extent of the addiction - diarrhoea, nausea, hypo/hyper-tension, hot/cold sweats, list goes on!
Good luck though man! Keep with it!