This is a wrapper for launching powershell scripts using shebang on WSL. Put the name of your user's directory into the second line, save this file into /usr/bin and make it executable.

powershell_wrapper.sh

#!/bin/bash
YOUR_WINDOWS_USERDIRECTORY_NAME_HERE=""
WIN_TEMP_PATH="/mnt/c/Users/$YOUR_WINDOWS_USERDIRECTORY_NAME_HERE/AppData/Local/Temp"

CURRENT_DIR=`pwd`

if [ $# -eq 0 ]
  then
    echo "No arguments supplied"
    exit 1
fi

SCRIPT_PATH="$1"

if ! [[ "$SCRIPT_PATH" == \/* ]] ;
then
        # relative path -> absolute path
        SCRIPT_PATH="$CURRENT_DIR/$SCRIPT_PATH"
fi

FILENAME="`basename $SCRIPT_PATH`"

if ! [[ "$SCRIPT_PATH" == \/mnt\/c\/* ]] ;
then
        # move to %TEMP% where powershell can use it
        cp "$SCRIPT_PATH" "$WIN_TEMP_PATH/"
        SCRIPT_PATH="$WIN_TEMP_PATH/$FILENAME"

        # add .ps1 if necessary - this only happens when we run a script from WSL
        # when we have files in windows land, they should have a ps1 extension anyway because windows likes its extensions
        if ! [[ "$FILENAME" == *\.ps1 ]] ;
        then
                mv "$SCRIPT_PATH" "$SCRIPT_PATH.ps1"
                SCRIPT_PATH="$SCRIPT_PATH.ps1"
        fi
fi

# substitute /mnt/c/ with C://
SCRIPT_PATH=${SCRIPT_PATH/\/mnt\/c/C\:\/\/}

/mnt/c/Windows/System32/WindowsPowerShell/v1.0/powershell.exe -File "$SCRIPT_PATH" ${@:2}