I recently discovered my NEW default sleeping kit. It is an ALPS Mountaineering, rectangular, micro fiber, sleeping bag liner. It measures 76 cm x 203 cm (30 inches by 80 inches) and weighs 450 gm (15.9 oz) in the stuff sack. It has a #5 YKK FULL SEPARATING zipper, so it can open and be used as a blanket or ground cover. Here is the web site:
http://www.alpsmountaineering.com/alps/products/bags/sleeping-bag-liners/rectangle-liner
I found it on Amazon.com for USD 21.00 (plus or minus).
As I am currently "between Caminos" the best way for me to test this was by sleeping out on one of my balconies (I live in an 8th floor flat). So, I did. It does what microfiber is intended to do. It works. Also, it is machine washable.
There is no insulation, per se, but outside at this time of year, the night temperature is running about 13 - 14 degrees (55 - 58 Fahrenheit). I was not uncomfortable.
This liner is just that, a sleeping bag liner. But for someone who can reliably expect they will have at least an albergue roof over their head each night while on Camino, this might be just the trick.
My biggest problem for three years running was finding a microfiber liner that was large enough for my "stout" frame; 106 Kg and 182 cm; AND which had a zipper that made attending to my frequent night time loo calls easier. I am over 60 and need to go to the toilet about every two hours. Suffice it to say, and bordering on TMI here, I think I found a solution, at least for warm weather Caminos.
Although I think that, at the price these things sell for, I suppose you could nest two inside each other and connect the zips with a twist tie or nylon wire tie so they open together. Otherwise, I would NEVER get where I needed to be fast enough...
OR if the liner were not warm enough you could choose to either use your opened poncho as a top cover, or simply wear clothing to sleep, or both.
Check it out. The liner also comes in a mummy shape that weighs fractionally less. But, for me, the rectangular liner seems to tick all the boxes, and the price is certainly right.
I hope this helps.