Thermolite sleeping bag liner. OK to use w/o bag?

I would say yes. I walked the Camino last July and August and did not see the need for a sleeping bag 99% of the time. The liner is a lot smaller and lighter and would be perfectly adequate. The albergues I stayed in were all heated and not at all cold, drafty buildings and a lot of nights on the Camino were not all that chilly that time of year.
I think the most important role the liner would play is hygienic as well. Keeping you off the mattress, although most albergues give you a disposabal mattress cover when you check in.

@musicagl - did you take the Reactor in the end, or did you take a sleeping bag? My down bag is very light but my liner which is similar to the Reactor but also insect repellent is less than half the weight and very comfortable (I am sitting in it now). I normally walk in September but was hoping to be able to do a week or two this summer with minimal pack weight. Also anyone else who has done the Camino with a polyester type bag instead of cotton or silk - what are the pros and cons?