The most compressible down layer

check out montbell.com

their down clothing is very light and compressible

Most down jackets I am familiar with easily compress to a size significantly smaller than a pillow–grapefruit size is more like it for some. The WM Flight or Nunatak Skaha STUFF down to the size of two grapefruits, for example. Put them in a compression sack and a 6" x6" x 7" ball is what you get. A comparable down vest would generally stuff down to a smaller size. For instance, a TNF Nuptse vest I have stuffs into its own pocket, which is about 9" x 5" x 6". I'd think that any down jacket made of UL materials and using high f.p. down would have a small compressed size.

I think you'd better be more specific, though. The down jackets I am referring to above are summer-three season appropriate, and while they'd certainly help on a snowshoeing day hike, they are not winter jackets and will not cut it in sub zero conditions. Maybe take a look at the Feathered Friends Volant jacket or Frontpoint jacket. I have a Frontpoint in Flite nylon and although its stuffed size is about 16" x 8" x 8", it compresses to about half that size.

"I was thinking that I could throw on this layer to keep warm during extended rest stops"

If you truly want some people to recomend some jackets, I think it would be best to give some temperature ranges and anticipated weather.

The "most compressible down jacket" will be the one with the least/lightest material and least down. I have 3 down jackets and while my montbell half sleeve weighs about 7 oz and compresses to the size of an apple, it is far from being acceptable for a Canadian winter. My TNF Prism tips the scale at 26 oz, but can be worn at any temp my winter throws at me.

I was thinking of something to wear when the temp is at freezing (0 deg C) and below (-10/15 deg C).

Thanks for the prompt.