Getting back to the question that so far has gone unanswered: you should be able to use silicone trivets, colanders and the like in the pressure cooker. At 15 psi, if I'm reading the literature correctly, the boiling point of water goes up to 250F at sea level (less if, say, you're in Denver). Serious Eats has a discussion about it, and there are other such discussions as well. Silicone utensils should be good to much hotter temperatures. I've noted a bit of discoloration on my silicone dish covers when roasted in the 400F range, but no melting. Webrestaurantstore says that silicone utensils should be good to 428F. That's a pretty wide safety margin above the pressure cooker temperatures.
Edited to add: those temperatures are for the interior of the pot. I don't know how hot the inside bottom of the pot gets, where the surface is in contact with flame. Maybe you'd want a screen or standoff to keep the trivet / bowl / etc. from being in direct contact with that hot bottom surface.
