If you have a seniority number on the list you could potentially take a LOA and fly the 737 at Swoop. Maybe even become Captain relatively quickly. Then when you have enough seniority at mainline to hold the left seat, switch back.
That's a lot of ifs and maybes.
I'm trying to figure out if you're trolling, have an overactive imagination or are just innocently misinformed. As it stands right now, there is no clause to allow pilots to take an LOA from either WJ or Encore and go fly for Swoop. LOAs at WJ, generally, are very limited. There is/was a provision for WJ pilots to fly for Encore and not lose their seniority but that was previously negotiated between the pilots and the company (and I stand to be corrected, but I don't think a single WJ pilot took that route). Also, as it stands right now, there has been no information about WJ's proposed wages & working conditions at Swoop. The only place I've heard "65%" mentioned is this message board - so it must true
Negotiations between ALPA and WJ are ongoing. So, you can proselytize all you want about how you think seniority and LOAs will work, but until the negotiations are complete, no one really knows how it will work out.
It is entirely possible that the agreement will have provisions that either make Swoop a position bid (similar to Rouge at AC - though I'm not suggesting the purpose of Swoop and Rouge is the same thing) or do have provisions on how seniority will, or will not, carry over between it and mainline. I think what is fair to say is that if WJ does not offer fair wages and working conditions for flying Swoop 737s, ALPA will negotiate very restrictive movement, seniority and scope clauses so that unlike at Encore, moving/flowing to WestJet mainline is not an enticement for pilots to join Swoop.