In theory there should be some useful water in there. In practical application, I don't see how or why it would be helpful to you to do this.
First, that filter housing you are using is holding-- just a guess here, maybe 35-40 gallons of water in it. And it's got a bunch of stuff in it that you don't want in your pool. It would seem that to get useful water out of there you'd have to maintain the water chemistry properly in that barrel while it's in there, which may not be as easy as it seems given the bunch of nastiness in there and the temperature rise such a small amount of water may go through. Letting it settle for a day and trying to skim off the top, imagine you could get 20 gallons out of there that isn't too terrible looking-- even that seems like a high estimate... but if it was 20 gallons
And if you didn't mind the idea that you'd be putting some junk back into your pool that you just a day prior filtered out.. would all of this work be any easier than just turning your fill hose on for a few minutes while you are backwashing? What's 20 gallons of hose water cost you too? I think for me it's about 8 cents.
A typical garden hose on wide open does about 5 gals per minute, so if you set the hose running, then backwash, then turn off the fill hose, you might be close to breaking even on the whole deal.
It seems you'd be doing an awful lot of work for a few gallons of not so clean water.
Maybe you could trim your backwash hose to a shorter length so it's a little easier to do the backwashing?