Run the send of the fx loop to the input of the delay. Run the delay's output to one side of the Y-adapter, and the output of the fx loop pedal to the other side of the Y. Then plug the tail of the Y into the next pedal or into the amplifier. Leave the delay pedal always on and at your favorite setting. When you activate the fx loop, the instrument signal will send to the delay and then to the amp, like normal. When you bypass the loop, your amp will get both the clean signal and the rest of what the delay is putting out, since you are only bypassing the feed to the delay and not the delay's output (which is still live and feeding the Y, along with the clean). There you go.
Here's a photo of my version of this, but there's nothing special about these particular pedals. You can do this with any delay and fx loop pedal.

As you can see, nothing needs to to the return jack cuz the delay out remains connected to the amp via the Y cable. Basically this setup selects between feeding the amp and the pedal (which also feeds the amp), but never disconnects the delay so that all your precious analogulous repeats get to live out their relatively short (600 milliseconds or less) lives.
I'm sure someone could build a box that does this without the Y-cable, but haven't seen anything on the market. Get on it, peeps!