Can the envelopes in Renoise instruments follow (or be multiplicated by) velocity?
Is there only VEL->VOL?
The Velocity Tracker meta device can only control DSPs that are per channel, not per oscillator, right?
I’m not sure whether I should use this to play a piano multi-sample and have low cut-off on low velocities, because it will probably sound weird when you play more than one note.
Thanks a lot!! 
in such a case you should divide the notes of the piano on more than one track, possibly grouping these tracks in a track group (available since v.2.8).
if you put the lower notes into a track and only there you put the Keyboard Tracker and the filter, only the notes on that track will be affected
Hmm… This sounds complicated for what I want to achieve. I should explain it differently, actually I want to play a piano sound live and I didn’t like the VST-Instruments I tried. So, I thought I’m going to build an XRNI that I like more, but it would probably need to keyboard track a filter cut-off so that silent notes become dull. That way I could build a piano sound without velocity layers, which are so much work. ATM I like this idea. But in general I just wanted to make velocity sensitive sounds without much work. And… VEL->VOL is okay, but soundwise it’s so “obvious”.
I see that for editing music the channel-wise solution would be doable, but I don’t see how I could prepare Renoise to actually keyboard track per oscillator/voice (like most software samplers do afaik) for playing the XRNI live on a master keyboard.
the registered version of Renoise comes with a VST recorder called Plugin Grabber which could be useful for your needs, as it can automatically record a VST into a multilayered XRNI: just select the note range and the number of layers and the XRNI will be automatically created.
more info here