alt Hacker NewsI don't know if it will work for booting Windows, but have you tried installing Haiku's BootManager to the MBR?
https://www.haiku-os.org/docs/userguide/en/applications/boot...
On the booting Windows front, this thread has a few people describing how they successfully used BootManager to dual and triple boot Haiku, Windows 7, and/or Linux:
https://discuss.haiku-os.org/t/dual-booting-haiku-and-window...
I have a laptop (Lenovo ThinkPad T410s) with Haiku, Linux Mint, and Devuan GNU/Linux. I use BootManager in the MBR, and it gives me a menu of all three to choose from. For the two Linux options, I have GRUB installed on each of their own partitions, so BootManager just sends me to the GRUB installed on whichever Linux partition. (Of course, this isn't exactly "replacing" GRUB altogether.)
I do it this way because I find dealing with BootManager to send me to individual GRUBs to boot a particular Linux partition much, much easier than fiddling with GRUB to boot Haiku.
(One last thought, parenthetical as I'm not sure it can even work from a USB drive: Worst case, you might make a USB "boot stick" that uses BootManager to choose between Haiku and Linux and let Windows do its jolly thing on the MBR?)
I tried to install the BootManager, but the limitations mentioned in the docs blocked the install. I think this part:
> BootManager isn't yet tested very well and still has a few restrictions that it will complain about if they aren't met: the menu can only be installed on your first harddisk and there has to be a 2KiB space after the Master Boot Record (MBR).
No idea how to create those conditions, the 2KiB space, and since I would have to do it with gparted in Linux anyway this made me install Linux instead first (with grub).
BootManager actually sounded nice - like you describe, I expected it to simply offer me the other OS(s) option(s), and I wasn't even set on installing Linux in the first place on that machine.
To your parenthesis: Yeah, I even had a boot manager on USB once for my PC, so it crossed my mind here, but for a travel laptop this would be a bit uncomfortable.