Hi kanito8a,
Cool, I have this game (played it a long time ago), but didn't have time to write a script for it ;)
Ok, let's start with script review, most of the script is good,, there's not much to say:
POL_Install_DosboxDrive
Don't know in what old script you found this statement, but it's totally obsolete for new DOSBox support.
Admittedly it's for now only documented in a hidden place.
imgmount d "$WINEPREFIX/drive_c/GOG Games/$INSTALL_DIR/$GAME_DIR/Anvil.dat" -t iso -fs iso
$GAME_DIR is never defined in your script
# DOSBOX CONFIG
cat <<_EOFCFG_ >> "$WINEPREFIX/playonlinux_dos.cfg"
...
Since you're using imgmount, manual_mount=true should be used in playonlinux_dos.cfg (and that will make imgmount -u unnecessary)
# CONVERT AND COPY ICON
convert "$WINEPREFIX/drive_c/GOG Games/$INSTALL_DIR/$GAME_DIR/gfw_high.ico" "$WINEPREFIX/drive_c/GOG Games/$INSTALL_DIR/$GAME_DIR/gfw.png"
cp "$WINEPREFIX/drive_c/GOG Games/$INSTALL_DIR/$GAME_DIR/gfw-5.png" "$REPERTOIRE/icones/32/$SHORTCUT_NAME"
Computing icons in script is depreciated, those days the recommended way is to get icons from the server (that's what the third parameter from POL_Shortcut is for); They're needed for repository lists anyway.
In my other scripts I didn't standardize on using gog.com's round icons, maybe all icons could be derived from your "top" gfx? I'm not sure Anvil of Dawn ever had any "official" icon...