I wonder if you just order the rail for the DD and have the Outback drilled and tapped to accept it?
AFAICT, the two rails have macine screws that pull the two rails together, not into anything between the barrels, but I might very well be wrong on that. The Outback has quite a space between the two barrels that runs pretty much entire length of the barrels. I believe it is wide enough for screws and plastic housing to fit into it at the smallest point in the gap, but on the DD maybe there is some metal all the way between the barrels and the rails screw into that. If the latter is the case, there's no way to use the DD's rails on the Outback (without doing crazy welding and drilling and tapping and there's no effing way I'm doing that).
You do what's best for you, but I'm not going to temper my purchases based on some ###### in Ottawa who doesn't know #### about guns. You do you, join the CCFR and fight for the right to enjoy it.
As for this project, your first scenario is incredibly unlikely in most of Canada unless you're dealing with the wrong crowd first. Since you're asking questions here, I'm hoping you're not

. Pretending that you already have the Outback, I think I'd contact Stoeger and ask if you can purchase their side rail separately. If that's not an option I'd look to the 3D printing world. I do a little bit at home here and the new machines are capable of printing quite well out of the box. A little bit of modeling in CAD and you could probably print something for you. Just a quick sketch;
https://ibb.co/Kb4gwmd
FWIW, I joined the CCFR just last week or the week before. I think they're doing a fantastic job. I hope that the banned list stops growing or the CCFR action kills what ignorant govt. ministers think wil make the world a better place, but, as much as I'd like to get the DD, just to be on the safe side, I'd buy the gun that will, again, be one of the last banned IF the list keeps on growing and the CCFR action does not succeed (I do not give a s..t WHAT the PM says; in Canada, politics and poltiicians like the PM defininitely DO "meddle" in the course of justice -- SNC Lavalin, for just one easy-to-remember example.)
Thanks for the printing offer (I think you offered, but "
you could probably print something for you" threw me for a bit of a loop). Nice preliminary sketch. If I can't find a solution via the rubber bands, I'll get back to you after I get the gun.
Speaking of which, first thing to get - the gun. Stoeger has a $50 coupon for purchases made before June 30.
BTW, I agee with you that the likelihood of a break-in is remote. That being said, however, our neighborhood's residents have had some thefts from their vehicles and our neighbor directly across the street had someone break into his home a few years ago while neither he nor his son were at home, and that someone stole some tools and hockey equipment. Of course that was pre-Covid days. (After that incident, some cops knocked on some doors in our neighborhood and warned people to be on the lookout. That was a bit of a jolt. "To be on the lookout" and do just exactly what?) There are now a lot of unemployed people and their perhaps-increasing economic desperation may just drive them to the point of breaking into homes. Who knows? My wife and I are retired and are home 98% of the time. Let's put it this way -- we're not getting any younger.
Back to the flashlight. I'm thinking that something like the following might be useable:
https://www.amazon.ca/Elzetta-Tactical-Shotgun-Flashlight-Mount/dp/B004R5V47C
From the photos, it looks like it installs without having to remove anything from the gun -- in that ad's case, a pump shotgun. It looks like you simply stack parts on either side of the (in an O/U's case) barrels, line up the holes, insert the screws and tighten everything together, against the barrels presumably.
But I'll start off with the rubber bands and take it from there.
Thanks very much to all of you for your suggestions and advice.