I have managed to add current time to VideoFullScreen in Estuary skin.
Script to do it is:
#!/bin/sh
echo "Deleting skin.estuary.edit"
rm -r /storage/.kodi/addons/skin.estuary.edit
cp -r /usr/share/kodi/addons/skin.estuary /storage/.kodi/addons/skin.estuary.edit
echo "Updating Addon.xml"
sed -i 's/id=\"skin.estuary\"/id=\"skin.estuary.edit\"/' /storage/.kodi/addons/skin.estuary.edit/addon.xml
sed -i 's/name=\"Estuary\"/name=\"Estuary (Edit)\"/' /storage/.kodi/addons/skin.estuary.edit/addon.xml
echo "Modified Addon.xml:"
cat /storage/.kodi/addons/skin.estuary.edit/addon.xml
echo "Creating temp TIME file"
rm /storage/.kodi/addons/skin.estuary.edit/sed.txt
touch /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo " <control type=\"label\" id=\"1\">" >> /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo " <font>font_clock</font>" >> /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo " <shadowcolor>black</shadowcolor>" >> /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo " <top>0</top>" >> /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo " <align>right</align>" >> /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo " <right>20</right>" >> /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo " <height>200</height>" >> /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo " <width>200</width>" >> /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo " <label>\$INFO[System.Time]</label>" >> /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo " </control>" >> /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo "Adding the TIME to VideoFullScreen.xml"
sed -i '/<controls>/r /storage/.kodi/addons/skin.estuary.edit/sed.txt' /storage/.kodi/addons/skin.estuary.edit/xml/VideoFullScreen.xml
echo "Deleting temp TIME file"
rm /storage/.kodi/addons/skin.estuary.edit/sed.txt
echo
echo "Modified VideoFullScreen.xml:"
cat /storage/.kodi/addons/skin.estuary.edit/xml/VideoFullScreen.xml
echo "The skin should be reloaded now ! (Choose different skin and then select this one again)"
When it is done for the first time it is necessary to reboot KODI and enable new (copied) skin in Addons (Look and feel -> Skin)