Hi Jimmy, I did in fact do that a couple of times. More often though I actually put one of my Billingham camera bags in to act as a day bag. I find that it works really well when travelling. Especially if you have to put your main bag in overhead storage or in a luggage rack on a train - you can still quickly and easily remove your valuables. The GR2 is easily big enough to accommodate either an Echo or a small shoulder bag.
I still have my GR2 but no longer have the Echo. I’ve thought about picking up one of the Bullet rucks for the same purpose recently – as a small day/overflow pack.
Hope that helps!