You're right, I don't want to change defaults. In fact, this whole back and forth reminded me that AutoHotKey exists, and I'm actually running it already, and I have the power to make this change myself. So I did!

    #if IsExcelFormulaBox()                ; Whenever the formula edit box has focus
    Tab::Send {Space}{Space}{Space}{Space} ; insert four spaces when I hit [Tab]
    $!Enter::Send {Enter}                  ; commit the formula with [Alt]+[Enter]
    $Enter::Send !{Enter}                  ; insert a newline with bare [Enter]
    #if

does what I want, with the helper function:

    IsExcelFormulaBox() {
        ControlGetFocus, F, A
        return (F="EXCEL<1")
    }