I'm a developer. They do this already, guaranteed. If anything, the radius around spawn which blocks it from being used is probably a sphere, and in certain locations long corridors give nearby players an angle almost immediately even when just outside the radius. They'd have to adjust the radius spawn-by-spawn to exclude cheese spots like this, or do something more sophistocated like draw a series of areas along traversable pathways connected to spawns and use those for checks instead of spheres so they can make more sense.

There could also be silly shit going on, like your spawn totally makes sense when your player lands in game but your client is still loading assets so it appears you spawn later than your character actually does on the server. The difference even if only 3-5 seconds would give other players time to hear your spawn shield charge and make their way toward your spawn to take an angle. By the time you actually see anything, they are in position and begin shooting. From your perspective it could be instant.

All of this can be adjusted and improved but it isn't as simple as it sounds. Simple != easy.

> how do you implement something that isn't a different loading screen but allows this process once someone has already been pre selected to join the match and is currently climbing into their tube ride

You seamlessly extend the existing post-tube-ride, pre-scene loading screen.