I would go for a UL synthetic quilt over a 35d UL down bag.
Quilt for summer, both for spg, fall, and winter.
I also had a bpl 90 and it definately wont fit over even a skinny bag. I also felt for a purely summer bag the foot box was sewn way too far up.
IMO you really want something you can open up almost to the feet. I sent mine back.
If it were me, and I am headed in this direction, I would either make or have one of the guys here make a synthetic overquilt to go over a #4 or #5 montbell UL SS down or other bag like your Summerlite.
I have a Montbell UL SS #5 down long bag and it is sweet.
Weighs 1#-4oz. Think mine actually weighs 1#5oz.
Has a 40D rating and I had it down to 40 degrees F with only silk long johns on and was very comfortable sleeping in the open with a a lot of dew and humidity. The only complaint I have about the #5 is that it does not have a zipper baffle. The #4 does have a zipper baffle.
Very roomy and light.
I also have montbell sythetic #3 and #7, both slightly on the heavy side for doubling up but better for high humidity.
All of the montbell SS bags zip pretty far down into the foot area if you want to use them as a quilt in the summer.
That said the #3, and #4 are too warm for me in the summer unless I opened them up and hung my feet out, then still too hot. The #5 is better and tolerable. The #7 is fine for a summer bag.
Right now I really have too many bags as I have 3 more besides those and will probably sell a few.
What I am working on right now might work for you. I plan to build a 50dF UL synthetic quilt with an open foot box, IE you can open the entire quilt up flat, or you can use with a closed footbox, but like a tube with the foot end open, or drawstring it closed, or variation of above. Got the idea from someone here that builds quilts and posted a photo of one with a drawstring foot box. Not sure if his opens completely up though.
Caught a sale on at joanns and I just put a test bag together to work out all the bugs and make sure I get the size right for under $20. Got 3/8" poly fiber for $6, some very light 2nds ripstop on sale at walmart for $7 for 5 yards, and vecro for $3. Weighs 13 oz. Figure that one should be good for about 60 degrees. Okay since I live almost in the tropics. I will post a pic of it later.
After I work out the bugs on the test model, next up is a DWR fabric version. It will be a .6 loft climasheild and should also weigh about 13 oz also.
If you combine a montbell #4 UL SS down bag with a 13 oz 50d quilt, the combo should be good for about 20-25dF and would weigh about 2#-4oz. Add a neoair regular and you are at 3#-2oz.
If you have someone build you a custom quilt, you can set it up however you want.