Is there a way to copy a particular sample from one instrument to another in the same song, or alternatively to make a second instrument point to the same sample?
It seems like the obvious way would be to drag a sample from one instrument to another, but of course you can only see the samples from one currently-selected instrument at a time, and dragging the sample onto the instrument name doesn’t appear to do anything.
I’m pretty sure you can just right click and copy/paste the sample from one slot to another… you may have to do the copy/paste with the sample editor.
I see what you mean – you can select the source sample, go into the Sample Editor, select all, copy, then switch to the destination instrument, create an empty sample slot, paste in the data in the Sample Editor.
I was thinking too much in Impulse Tracker terms where the samples occupy a global namespace and are merely “referenced” by the instruments, rather than in Renoise terms where samples are “contained” within an instrument.
It does of course mean that the sample data is duplicated, but this is not a problem for me as I am only doing this to set up instruments (before saving them to disk) rather than within an actual song.
easiest way:
go to sample editor
select sample/instrument you want (if it’s a sub intrument, doubleclick so that the actual sample turns gray)
cmd/ctrl+c to copy (the sample editor marks the full sample)
select the new sample/instrument location
cmd/ctrl+v to paste in the new location.
if you want to copy the whole drumkit/instrument, make sure your initial sample is marked orange before you copy/paste.
way more effective to learn how the keyboard works, instead of messing with that mouse
edit: the colors reflect the default theme 
in addition, i would like to know if it is possible to somehow, with the push of just one button, rename all instruments so they are called “goat”? if not, i would want that.
you could easily make an XRNS-SF script for that…
+1 (every day problem for me, just not enough goat named instruments and no easy way of making any)