As long as the canister was being fed as much water as it was pumping there would be no problem in the overflow...however I would not recommend it. In essence a canister filter is a closed loop and must remain closed to work properly. Unless you're sucking from the bottom/middle and not the top, then there is the possibility of breaking the loop and burning out your pump.

I use a canister filter for water movement and when necessairy (a lot lately) mechanical and chemical filtration (e.g. filter pads and carbon). Otherwise I just remove them and let it run. (I use that instead of a big  pump for a closed loop as it's what I already owned, and have other plans for the $$$)

What is the expetation of the canister filter? Are you targeting a specific contaminant? Or, are you trying to just get rid of foaties? Possibly the more the merrier thought?

I would say that ensuring your pump is properly supplied, size your pump accordingly to the maximum volume capacity of the discharge side plumbing. Will you be using a pump on both sides? If so, make them twins, and you shouldn't have issues. Or, use a sumbersible where it's flooded.

Don't most overflows allow some air into them? From what I understand air+canisters=noise and .

It could work though if the rate that water came into the overflow was the same(or a little more) as the canister's flow.

If you're running a sump mabe try running the intake of the canister from the sump and the output up to the main tank similar to what gonzo said.