Is there any version of GRUB that dynamically detects what OSes it can boot on the current disk(s), so that I don't have to run
update-grub every single time some OS changes on my machine?
I don't know of any GRUB variant which doesn't require some sort of on-disk configuration, but you might want to look into alternatives. I personally use Chameleon ( http://chameleon.osx86.hu/ ), since it is capable of booting OS X, chainloading to the Windows boot manager and multiple linux distributions. It auto-detects on boot.
However, a better fit for your needs would probably be something like the rEFInd Boot Manager ( http://www.rodsbooks.com/refind/ ), a graphical boot manager with auto-detection of EFI and BIOS boot loaders.