I would be concerned that the burn string, would not burn when the engine ejects. Instead, I'd work up a way for the motor to hold the string in place when the motor is in the mount, and the string free to slide out or otherwise be released once the casing is no longer there. But I'd rather just keep the engine in it for simpicity/reliaiblity.
Take note that most Rocket Rocket Glider type designs are all designed for the burned out engine mass to be up front as noseweight. You don't want to eject the casing, only to have to add noseweight of the equivalent mass. I did one swing-wing boost glider, with a pop-pod. Just one. And it needed a lot of noseweight.
I'll also note that for nearly all models I fly that use burning "string", I use elastic thread. So I can pull it taut for boost, and when it burns it "snaps" itself out of the way.
Ironically, I would expect greater reliability issues with NOT using a burn thread, than using a system depending on the engine casing ejecting itself, where the thread may end up getting hung up on something (or not burning if burning is tried).
And that's not even in the top 2 of swing-wing problems. #1 is deployment strength (as in not enough), and #2 is hinge weakness/damage. Also a lurking problem is engine exhaust burning the string/ thread during launch.
Here is a thread I created 10 years ago that describes some methods for strong hinges (yet allowing the wings to be removed easily), for better rubber band leverage, and the method I used to solve the problem of engine exhaust burning the thread (wings are locked by a music wire pin on boost, that is pulled out to unlock them once a burn thread allows a rubber band and unburnable cord to pull the pin out).
https://www.rocketryforum.com/threads/swing-wing-gliders-wing-hinges-and-big-models-too.18665/
Below - 6 foot span swingwings, 1972 , and 2008.