The chroot methoed (mount .img file, chroot into it) works exactly like that (sorta)
instead of a reboot, you just need to run the script to load BT. it boots it up into a chroot environment with leaving the phone still essentially in control. you can take and make calls and texts, surf the web, even play games with it running in he background.
the only downfall is a lack of a custom kernel and lack of control over devices, as the phone OS still essentially runs everything (wifi needs to be on phone side for it too show in BT, so on and so forth)