I'd be afraid that a bag fine enough to hold in the hop pellets would reduce the flow too much. You want the hot wort to flow over the hops. If you just put the hops in a bag and put that inside a hopback, I don't think much wort would flow through it. It would flow around it. You'd have to direct the flow into the bag, and I think it would end up reducing flow too much. If you are thinking of something analogous to the coffee singles, remember the coffee for that is ground relatively coarsely. Have you ever tried using espresso gound coffee in a drip coffee maker? Instant plug (been there, done that).