Strangely enough, I think you're the first one to point it out. But it makes perfect sense to have this as a native feature.
And yes, it can worked around using tools. But the one I'm thinking of (Duplex) does seem like overkill for this particular scenario.
So, I've actually started working on a MIDI remapping tool which would enable this sort of thing - while still being light on CPU and everything.
The idea is to rewrite incoming MIDI on the fly, using "rules" like this one:
WHEN channel is between 1 and 16
THEN set channel to 1
In theory, you could even produce a rule which would broadcast incoming message on all channels :-D
What is particularly nice is that, with such a tool, any existing mappings should continue to work. This is so, because Renoise doesn't really care which MIDI device the input originally came from, it just cares about the messages themselves.
I started writing the tool a couple of days ago, and there's still some finetuning to do. Will let you know when there's something to test.