The Hollis Katana is a nicely streamlined until that offers 35 pounds of lift. It's enough for a pair of 100's, LP 85's, LP95s, HP 130s etc, but not too much to make it difficult to travel with.
The Dive Rite Nomad LS has the same 35 pound lift capacity and also travels well, and it trims very well for pretty much anyone. It comes with a standard bungee system but is very easy to convert to a cleaner loop bungee configuration.
The Scubapro side mount system is very comfortable and that might be an added plus with a bad back. It can be obtained with a 25 pound or 45 pound wing. The 25 pound might not be enough for trouble 3442 psi 100s, but it will depend on the person, their exposure suit, etc. However, given that she is diving dry, 25 pounds should be plenty, and would be ideal for AL80s and travel purposes. The 45 pound wing will be more than adequate, but the wing will "turtle shell" a bit more than a smaller wing, however that's really an over stated concern as the turtling I've observed with it has been very streamlined and it dives very clean in the water with very low drag. And if you're in serious sidemount passage anyway you end up dumping the wong to reduce volume anyway. The Scuba pro system trims out extremely well and is very tolerant of lateral weight differences. The downside with the Scubapro wing is that it won't pack as small. The included bungee can be re-rigged as a loop bungee.
As noted above the X-Deep Stealth is an option, but some filks find the shoulder harness to be a little restrictive. The bottom of the shoulder straps attach to a fabric plate in the small of the back and they connect above the waist belt, so there's a bit less room than with a H harness or with a v style harness that has the shoulder straps attaching at the waist.
----
AL 80s are your best bet for single tank monkey diving, but be aware you'll need to adjust the hose routing to ensure you have an alternate air source for the single tank dives. An Air 2, Tusa Duo Air, etc, works fine is you are not doing serious sidemount dives, and it makes the hose routing very clean with no need to change the hoses on one of your regs every time you switch from a single to dual tank configuration.
In some rigs you may find the lateral trim is an issue given the asymmetric weighting caused by a single tank. AL 80s start out a couple pounds negative when full with a reg attached and then get a couple pounds light when near empty. Some folks just add a pound or two of weight to the non tank side of the harness to split the difference, while others use a 2-3 pound weight and shift it from a pocket on the non tank side to a centerline or tank side pocket as the tank gets lighter.
-----
For travel purposes, I suggest getting a pair of compact "1 1/2"" cam bands to secure the lower bolt snaps to the tanks. You can use 550 para cord tied in a loop with an over hand knot and fed under the loop of cam band in a tri-glide to secure it to the cam band. Leave the loop long and you can pull it back and forth to get the proper tail length, then reposition the knot and trim off the excess.
You can also use para cord to create a loop about 6" long tied through the eye of a bold snap and then larks headed around the base of the tank valve to all a "hard" connection between the tank valve and the shoulder D-rings to support the tanks if the loop bungee breaks and to allow the tanks to be supported if you ever carry then out of the water.
I use 1" x 8" or 1 x 8 1/2" 'Dally wrap' to hold the hoses against the tank. Dally wrap is basically high grade rubber inner tube in 1" or 1.5" width that is used by ropers to wrap saddle horns. You can find it on amazon in packages of $10 or 12 for around $14-15 - about 1/4th the cost they sell it for in dive shops.
When traveling I use the tank neck bolt snap and cord looped through the dally wrap and cam band to hold the set together.
