I've never been successful at selecting a WestJet seat on a DL ticketed itinerary.

However, I do connections through LGA on a WestJet flight that was ticketed by DL all the time. I can check in online or at a kiosk the origin aiport through DL and print boarding passes, but I have to get a new BP at the WestJet gate in LGA. They will usually let me pick an exit row then as well.

The other direction, YYZ-LGA-??, I have to check in at the WestJet gate. I get both BPs but I usually have the DL gate reprint the pass as the DL barcode reader doesn't like the WestJet BP.

However, be very careful with SDCing or changing to a different WestJet flight for any reason, if you do it through WestJet ticket counter or gate agent. I did that in late December and had to jump through multiple hoops with Delta to get MQM credit. I'd recommend calling Delta and having them do the change.

If you are successful in preselecting a WestJet seat, please post what you did.