Interesting. Not an expert in sidemount so can't comment on the waist strap bladder issue. I dove the SMS 50 (Sport) with 7mm wet suit and also dry suit (with AL80s only), and they will trim out fine with shoulder weights. It's a fairly standard solution. If lift wasn't a problem (I'm diving LP85s and AL40 deco bottle in 40s water), I'd still be diving the SMS 50 which is very travel friendly. Before getting the SMS 100, I weighed the option of the SMS 75. I like it a lot but its lift is still borderline for cold water diving, IMO. Hence the SMS 100. I implemented some of Edd's mods (esp. tightening the upper part of the bladder so that left biases toward the hip area) which works fine. Being a bit bulky is its main drawback. Not going on the plane as carry on.
As you probably surmised, the instructor for my class was Edd. Lift was indeed a problem for me with the SMS 50. I was not going to be putting any lead in the shoulder with LP 85s, believe me. Of course, it depends upon which brand of LP 85 you are talking about. I own Worthingtons. Fabers are lighter.