To help answer your question, could you explain why you don't want to go with high loft insulation?
Wouldn't 3 to 5 oz of insulation more in your sleeping bag be warmer than simply carrying a 3-5oz liner? (I have a coccoon liner that I have used to add warth to a bag, but no longer using it, having bought 850 down fill bag that is 15 degree at 2lb 3 oz.).
Anyway, to add my thoughts to your question, I would think light weight therma tops and bottoms would help out.
I recently purchased BPL's Merino wool long johns & long sleeve shirt to help me increase the warmth of my 40 degree Marmot Atom bag, but I have yet to try out this combination.
Sleeping with a fleece hat will help.
I have slept with my windshirt on to add warmth, but I do find that since it does not breath well, it can get a little humid/uncomfortable.
For me, the biggest enhancement has been laying my jacket across my chest like a blanket inside my sleeping bag to capture more warmth. In my case, I have a Montbell Thermawrap Jacket.
Hope this helps.