@Leos KlienI was thinking about that as well, but then I thought of a few things:
1 Heatran is a LAVA dome pokemon
2 He already has extreme heat resistance
3 According to the pokedex, '
Boiling blood, like magma, circulates through its body. It makes its dwelling place in volcanic caves.' If his Legendary pokemon is so hot that magma is inside him, why not he himself?
4 Flash fire ability
5 Even other fire type pokemons who don't have abilities like flash fire can survive lava, for example in the anime where charizard and magmar literally dived into a vulcano and came back out not long after, or pokemon mystery dungeon (I know it works like this in explorers of darkness at least) where every fire type can literally walk over lava and take no damage at all. If they can, I think a legendary fire/steel type pokemon and his avatar should also be able to.
EDIT: I took so long listing this you already admitted I was right before I posted it xD