An upside aluminium tray is placed directly over the flame on the blech and reaches yad soledes. Is one allowed to place a chalah on top of this tray in order to warm up the chalah.
Some permit the reheating of cold and dry food that is fully cooked on top of an inverted pot. Although other authorities are stringent, requiring specifically a full pot of food, many are lenient to follow the former opinion. Therefore, it is permitted to heat a challah on top of the tray.
Sources: Concerning using an upside down tray/pot in general, see Shemiras Shabbos Kehilchasah note 112; Shevet Halevi 1:91; and see also http://www.dinonline.org/2011/02/03/shabbos-hatmana-returning-food-to-heat-on-shabbos/, note 12-13. Concerning heating a challah, see Yevakesh Torah, Shabbos, p. 172, who writes at length concerning whether one must be concerned for the hardening of the challah (and its becoming toast-like), and concludes that one may be lenient in this matter.