Huh? I've dual-booted every Chromebook I've had without any disassembly
For one, you'd never flash the BIOS on a Chromebook, or really any modern laptop; most laptops use UEFI instead, while Chromebooks use Coreboot.
For two, once you flip on the Developer option, you get the nag screen about security . . . and you can now boot from devices via a "Legacy Boot" option provided by a SeaBIOS instance shipped with the Coreboot firmware to boot other OSes. Every single Chromebook I've ever had, I've dual-booted with a standard Linux distro this way. I've never opened up any of them.
There are also other ways, like packaging a Linux distro up as a ChromeOS image, and I also think many newer Chromebooks ship with a UEFI layer too that can be used in Developer mode? I've never tried either of those two approaches myself though.
You only need to do any sort of screw removal if you want to actually flash your own copy of the firmware. Which matters for people who want 100% Free Software running on their hardware and thus want to flash Libreboot, or have some other very very geeky reason to want to run your own firmware on the board (ex. trying to add stuff so that Windows will deign to run on it), but not so much for anyone else.