I have confirmed that if the mayor dies and a new one is re-elected all the positions reappear. I have not been able to confirm that this will happen for expedition leaders. For small fortresses if you change the line in the entity_default.txt raw file under mayor from [REQUIRES_POPULATION:50] to [REQUIRES_POPULATION:1] you should be guaranteed to have a mayor at all times and therefore only suffer a season or so downtime until the positions become available again.
I haven't tested this yet so if anyone does this and the raw change doesn't work as expected please post a seperate bug report about the raws not working. If expeditions leaders do in fact also get re-elected then make a note of that here.
Seems to work on a new worldgen/embark, after having my expedition leader becoming mayor as soon as the game started (expedition leader always become the 1st mayor), i walled my military (a miner and the mayor) in a room with damp stone, disbanded the military and dug the damp stone wall.
Got the mayor drowning then.
As expected the bug when expedition leader/mayor dies occured, no more noble role listed.
I waited and waited, then a new mayor was elected to replace the original expedition leader/mayor and the entries came back.
Now that was on a completely new game, for an already exisiting one, saved after you lost the expedition leader, modifiying the in both the \raw\objects\ and the saved game...regionX\raw\objects\ changed nothing, after 2 seasons always no entry anywhere in the noble menu.
So lesson to learn before playing DF2010 : modify the line suggested by Drunken or as soon as you lose the expedition leader, your fun will be broken due to every noble entry exisiting only if that guy is alive.
And even hiding the expedition leader in a deep underground burrow guarded by dozen of guards will not be enough, as it needs just a "mood" that can't be satisfied to get him insane then dead, leading into the fun ending.
And with the mayor system started, losing the expedition leader (the 1st mayor) will not break the game anymore, as the noble listing will come back at each new mayor elected.
The point is to have the expedition leader surviving enough to reach the mayor requirement, and you're safe.