Well, Debian has a much bigger community, users and developers that are using armhf or other platforms. Then it's much simpler to support.
For VM builds, you'd have to use --arch and maybe --kernel in the build script. But that doesn't apply to physical isolation users. (Because then a base operating system is already installed.)
The build script might include 99% of what's included. But the missing 1% I know might totally break it.
In https://github.com/Whonix/Whonix/blob/master/aptrepo_local/conf/distributions we're using "i386 amd64". Using "all" isn't supported by reprepro.
Error: Distribution stable contains an architecture called 'all'.
But I guess it would be worth adding all the architectures that Debian supports and then see how it works for early adapters such as you.
Got any development / git knowledge?