conversion - How to write aligned equations and export them HTML/TeX

This question is mainly about exporting to HTML/TeX. For those of you who do not know how to write aligned equations in Mathematica please see this answer. Lets assume that you modified your stylesheet as I mentioned in my answer. In my notebook this is the cell expression I have:

Cell[BoxData[
    FormBox[GridBox[{
    {
    RowBox[{
    RowBox[{"a", "+", "b", "+", "c"}], "\[AlignmentMarker]", "=", "d"}]},
    {
    RowBox[{"c", "\[AlignmentMarker]", "=", 
    RowBox[{"d", "-", "a", "-", "b"}]}]}

}], TraditionalForm]], "DisplayMath"]

Now I want to export this expression to TeX, we can do this with:

cell = Cell[BoxData[
        FormBox[GridBox[{
        {
        RowBox[{
        RowBox[{"a", "+", "b", "+", "c"}], "\[AlignmentMarker]", "=", "d"}]},
        {
        RowBox[{"c", "\[AlignmentMarker]", "=", 

        RowBox[{"d", "-", "a", "-", "b"}]}]}
    }], TraditionalForm]], "DisplayMath"];
Convert`TeX`BoxesToTeX[cell]

The output is:

\begin{array}{c}
 a+b+c=d \\
 c=d-a-b
\end{array}

The function Convert`TeX`BoxesToTeX is undocumented but the documentation does show how to use it under examples. It seems that the function doesn't do anything with the \[AligmentMarker], but I'm sure we could make good use of them.

What we want to do is find any appearances of the \[AligmentMarker] and substitute them with "&". And if we did find some \[AligmentMaker]s then we use StringReplace to change "array" for "aligned". This is the output that I would like to end up with:

\begin{aligned}
 a+b+c&=d \\
 c&=d-a-b
\end{aligned}

Does anyone know how to create a wrapper function that does this? The main problem I'm having is replacing things in cell.