Sue, the more I've thought about this, the more I think that this cucumber must have used the 1" gap by the return to access the overflow -- at least, that makes the most sense to me -- I just do not think that it would want to claw out of the water.
I know how impossible it can be getting things like hands and nets into overflows. If you are able to get something into the area that the cucumber keeps crawling onto (whether it be a tube of some kind or even clamping some plexiglass into the corner), if the cuc would wander onto it, then perhaps you could get a large media bag (like those made to hold carbon) below the tube, in case the cuc lets go while you are trying to move it.
I just don't know how to "bait" a cucumber.... but I did manage to get a dottyback out of the small overflow area in my one 'Pod by submerging a large nylon media bag and feeding him -- and waiting...... until I could quickly close the drawstrings & voila! dottyback in a bag! I suppose, if the cuc was willing to climb over it, you could do something similar in your overflow. Perhaps lay some screening against the sides of the overflow and use the screen sort of like the tube -- or maybe just wad some up and place it in the overflow, in the hopes that the cuc will crawl over it and you can somehow "wrap" it w/ the screen and bring the entire thing up and out of the sump (since you could use tongs to grab at the screening.)
Just brainstorming here....