The BibTeX entry results returned from citation() place the R package version in the "note" field. It would allow for cleaner bibtex parsing if it was in the "version" field.
The "version" field is not used in standard styles. See Patashnik's original documentation in <http://mirrors.ctan.org/biblio/bibtex/base/btxdoc.pdf>, or a more recent description as in <https://en.wikibooks.org/wiki/LaTeX/Bibliography_Management#Entry_and_field_types_in_.bib_files>.
The format allows you to add extra fields and you can write styles to display them, but if R did that, its output wouldn't work for most people.
Whoops, "version" *is* included in my second reference. I missed it on first reading.
And after a little more research, I can't find any BibTeX styles that support "version". Only biblatex supports it, and R doesn't currently have any biblatex support. So I'll leave this open and on the wishlist, but it seems to me it's more appropriate for a contributed package author to write, rather than putting it into core R.