Not sure if you already did this, but the CO routing for CLE-SEA is:

ROUTING 422 *** TRAVEL MUST NOT BE NONSTOP ***
422 FROM CLE-
TO LAX-SEA*
OR TO HOU/CVG/DFW/SLC-SEA*

After running that through faremetrics.com, it gives:

Route Segments Mileage
CLE-LAX-SEA 2 3006
CLE-HOU-SEA 2 2999
CLE-IAH-SEA 2 2965
CLE-DFW-SEA 2 2691
CLE-CVG-SEA 2 2469
CLE-SLC-SEA 2 2252

I would try those connecting cities and see what you get. I also only searched for CO flights, not any partner flights which would allow you to get CO mileage credit, so that may open up some possibility if you investigate routings on alternate carriers. Good luck.