After much research (between the CCS Hybrid Pack), I've decided to go with the SealLine Pro 70L (it also happens to be 25% off on Amazon at the moment). I was curious if anyone had a system where they've added a pack of some sort (Fanny Pack type maybe?) or extra storage to the 4 clips on the back of the pack?

Yes, not on my Sealline but on a similar pack. I used it for the things I wanted to keep handy during the day. As long as I made sure to attach it in a way that it didn't flop around while portaging, it was fine. Test out attachment systems before you go. Being able to quickly clip or unclip it was important to it not getting annoying for me.
Although if it was a small bag and I wanted to keep it really close to me while in the boat, I usually just tucked it under the top straps of the pack...that's what I do with loose water bottles at portages as well.

What did you use for the extra storage? I have a 20L day pack that I think would fit perfect, and not flail around on the portages.

That’s a good looking bag. Man. Real nice. I have an old 115 sea line bag that’s still going strong.

With those daisy chains down each side you will be able to rig something for sure. Maybe look at those recreational barrel works saddle bags. They might fit the bill.

ccslick: "After much research (between the CCS Hybrid Pack), I've decided to go with the SealLine Pro 70L (it also happens to be 25% off on Amazon at the moment). I was curious if anyone had a system where they've added a pack of some sort (Fanny Pack type maybe?) or extra storage to the 4 clips on the back of the pack?"

Am I the only one that puts my 30L day pack on my front? I don't fill it too full and lean forward slightly due to counterbalance my main pack so seeing my feet has never been an issue. It also has a chest strap that really helps with keeping it from pinching my arms so it is comfortable. Much better than attaching that weight to the outside of a main pack and moving my center of gravity further back.

A1t2o: "Am I the only one that puts my 30L day pack on my front? I don't fill it too full and lean forward slightly due to counterbalance my main pack so seeing my feet has never been an issue. It also has a chest strap that really helps with keeping it from pinching my arms so it is comfortable. Much better than attaching that weight to the outside of a main pack and moving my center of gravity further back."

I guess I've never thought of that. The only issue I see is that we single portage as a group, and the one without the canoe carries the food pack on their front. So I'd be curious to see if front carrying a day pack would be doable with that pack and canoe. I like the idea!

ccslick: "

So I'd be curious to see if front carrying a day pack would be doable with that pack and canoe. I like the idea!"

I single portage with a 70L internal frame backpack, 11.7L front pack or canister and canoe with two paddles and PFD.