Creating a rhombus is not a big deal. All you need is a line in 45 degrees with the right thickness.
This XML snippet creates an rhombus.
<draw name="Rhombus" y="25.4mm" x="28.575mm" w="20mm" h="20mm">
<value>
<line slope="/">
<edge cap="butt" thickness="28.28mm">
<color value="255,153,0"/>
</edge>
</line>
</value>
</draw>

However, since you can't create polygones in Designer you won't get a triangle that way.
But you can use a dingbat font which contains such symbol alternatively.