The only thing I've done that's close to a meatloaf is a fatty - but I'd have to agree with you that 95% lean hamburger might dry out if you're not careful. That's a bulky item you're smoking, so I'm guessing it'll have to be in for a while. Maybe the lean would be OK if you put foil under it to catch the fat that does come out, which will keep it around the base of the meatloaf? When I do a fatty, I take my rack out and put it on the coutertop. I take the foil and lay it on the area where the fatty goes, and then press the foil down between the grates down to the countertop on either side of wher the fatty will go, which makes a "fat trough" on either side of the fatty. Keeps the juice really close but keeps the meat from actually sitting in it. I've got one in the smoker right now - if I had a camera handy I'd take a pic (wife is on vacation and has it).
I'd probably be inclined to spritz it pretty regularly too.
Maybe you could do like SmokyOky does his briskets and sear it before you put it in the smoker? Kind of a wild idea for a meatloaf, I agree - and I'm not an expert on searing or what that might actually buy you with a meatloaf vs. a steak.
The meatloaf is on my wife's agenda for me to attempt in a few weeks, so let me know how this comes out - document what you can and post some pics if you can.