writeClipboard works for raw or character vectors. Is there anything that can let me place matrices/data.frames into the clipboard?
I think write.table with the file = "clipboard" argument will work.  Here's an example:
write.table(mtcars, "clipboard", sep="\t", row.names=FALSE)
                                                                        The above is mostly a wrong answer if your table is a little bigger.
Warning message:
In .External2(C_writetable, x, file, nrow(x), p, rnames, sep, eol,  :
  clipboard buffer is full and output lost
The hack fix is this:
writeClipboard(knitr::kable(d))
                                                                        A nice OS independent solution is:
library(clipr)
clipr::write_clip(mtcars)
But may suffer from size limitations as well.