I won't provide a patch because it's too trivial. Just add "luatex lualatex" to the #compdef line of zsh/Completion/Unix/Command/_tex
I have to admit I don't understand the difference between adding these commands and having an entry such as for xelatex which is xelatex=latex. Somethinge like this might be appropriate, too, since lualatex is the envisioned successor of pdflatex. For more information on LuaLaTeX, see http://www.luatex.org/. The two main features in short: native UTF 8 support and OpenType support, including advanced OpenType features. lualatex is even more of a drop-in replacement for pdflatex than xelatex is, so this should not be a problem.