Just a little idea…
Would it be possible to make/mod a linux distro, so that it almost only runs Renoise?
Without getting into the “why would you want this”…
You can but renoise will be a little useless unless you also are running alsa or better jack and probably qjackctl 
http://susestudio.com/ has been around for a bit and allows you to custom build live cds, you choose the distro and I believe you can upload installers (like renoise since there is no renoise repo) and install from script… just don’t distribute the live cd with your Reg’d Renoise or you’ll get your Renoise serial revoked.
There are a few distros out there that include renoise demo - avlinux and the kxstudio non-free repos host the demo binaries… but they have a lot more than just renoise.
Hmm… i suppose it’s a pointless idea 
I’ve always thought it’d be pretty cool to have a laptop or netbook running a tracker as the “only” program on the OS, lol.
It’s kind-of possible.
You can install a really stripped down distribution (I used ArchLinux for that) with only the absolute bare minimum to get X11 and jackd running. Then create an automatic logon script that logs on as a user, starts X11 and loads Renoise. You don’t need a window manager to run Renoise on Linux (although I’d recommend it if you are using VSTi’s / DSSI etc. with extra windows). The boot time is reasonably small and you can disable all other devices and daemons (networking etc.).
However, you can also do this with Windows to some extent (which is what I’ve done on my main PC). Use nLite to strip out the majority of Windows components and put a link to Renoise in the startup group. It will boot straight into Renoise. Save your work and just hit the power button to shutdown.
The difference between a stripped Linux and stripped Windows XP is not really noticable. Both boot really quickly and get you into Renoise from power off in ~10 seconds (on my old machines).
i have a renoise.desktop file in my crunchbang-linux install, which basically just starts openbox with Renoise running. i tried this out to cut down on system resources by software i do not need when running Renoise. but haven’t really tried very much.
I set this up using Win XP and a bat file so that when you close Renoise it shuts down the PC
C:\Program Files\Renoise 2.1.0\renoise.exe
shutdown -r
That was the contents of the bat file
Also rather than having Renoise set up in the start up folder i actually just set Renoise as my desktop, no start bar or anything
Run msconfig then click boot.ini to run in nogui boot mode to remove the crappy windows load up animation
Annoys me having the horrid blue windows welcome screen after it boots pre Renoise load though hahaha
And then of course the horrid blue xp closing down screen haha
I never did find out if you could change those 
Well i had to do a bit of research to see if this could be updated and it seems that Win 7 64 is even better for hacking this together, you can remove everything that identifies it as Windows, indeed you can even replace them haha
Seems i may have to get this going again
Oh for the record i made a super tiny cut down crunch bang linux that booted straight into Renoise, the reason it went back to windows was simple, more plugins and lower latency 
In KDE I had a login type that only loaded a window manager, sound, and Renoise, it’s simple to do once you know how the login process works.
I was trying to do this last month, but I just had a bitch of a time with Arch. 
I’ll probably try again at some point though, as I always liked the idea.