I am the original reporter of https://bugs.archlinux.org/task/25726 and as Desmond Cox explained, it is more practical to use chainloading than having one grub. Maintenance is more difficult when having several kernels from different distribution in one grub, and increases risks of having one distribution wrongly editing entries when updating their own kernel options.
Like Desmond, I never experienced any problem installing grub in a partition. Removing the possibility to install grub in a partition is in my opinion contrary to KISS, especially if there are no stable alternatives (I shall try syslinux, as you recommend, but other posts make me wonder if this would work for me). Is it possible to allow installing grub on partitions as an "advanced" option?
Thanks a lot in advance.