automatically inject code into preamble in cascading manner

Question

Is there a standard way to automatically inject certain files into a LaTeX preamble? Naturally I could write some patch code for this but I thought I should check if there's some "preferred" way to accomplish the job.

Inspired by Creating a default preamble the hypothetical MWE is as follows:

mypreamble.sty

\NeedsTeXFormat{LaTeX2e}[1994/06/01]
\ProvidesPackage{mypreamble}
 [2015/11/18 MyPreamble]
\RequirePackage[dvipsnames]{xcolor}
\endinput

file.tex

\documentclass{article}
\newcommand{\wow}[1]{\textbf{Wow, much #1!}}
\begin{document}
\textcolor{red}{This works, trust me.}
\wow{color}
\end{document}

Invocation would then be something like:

smart-pdflatex --with-style mypreamble.sty file.tex

Output to be the same as running

pdflatex smart-file.tex 

smart-file.tex

\documentclass{article}
\usepackage{mypreamble}
\newcommand{\wow}[1]{\textbf{Wow, much #1!}}
\begin{document}
\textcolor{red}{This works, trust me.}
\wow{color}
\end{document}

---

Note, the above examples use the article class, but I'm hoping for a solution that also works with memoir, etc.; and similarly, it would be great to have a recipe that works for xelatex as well as pdflatex.

At the moment the above examples are working only for the article×pdflatex "quadrant", using the recipe from David Carlisle's answer below. Switching article to memoir, I get

! LaTeX Error: Command \@namelet already defined.

And with xelatex I get

! LaTeX Error: Missing \begin{document}.

---

Update @cfr, Dec 19 2015, 0123 UTC:

For example, with mypreamble.sty as above, and adjusting the compilation command following suggestions from comments below, I try to compile

file.tex

\documentclass{memoir}
\newcommand{\wow}[1]{\textbf{Wow, much #1!}}
\begin{document}
\textcolor{red}{This works, trust me.}
\wow{color}
\end{document}

with the command (all on one line, no extra spaces):

pdflatex -jobname=file \\documentclass[a4paper,11pt]{memoir}\\usepackage{mypreamble}\\renewcommand\\documen‌​tclass[2][]{}\\input{file}

and I get the error

! LaTeX Error: \documen undefined.

Update - invisible character (oops)

The command above looks the same as this one but contains an invisible character, which is the source of the error. The following command works with memoir.

pdflatex -jobname=file \\documentclass[a4paper,11pt]{memoir}\\usepackage{mypreamble}\\renewcommand\\documentclass[2][]{}\\input{file}

Answer 1

pdflatex -jobname=file \\RequirePackage{mypreamble} \\input{file}

would work in the basic case (you may or may not need to double the \\ depending on your commandline shell)

If requirements vary (e.g. for xelatex+memoir), some adjustments are needed.

xelatex -jobname=file \\documentclass[a4paper,11pt]{memoir}\\usepackage{mypreamble}\\renewcommand\\documentclass[2][]{}\\input{file}