Maybe Bing Crosby signing his golf scorecard (a fine 78, almost shooting his age) and then dropping dead right off the 18th green, no fuss or muss, but that wasn't as cinematic. Lots of actors die in character if their characters are fat (James Gandolfini, John Candy) or fat and out of control (Chris Farley), but that's kind of depressing.
Looking it up, it appears there are numerous theories that Howard's 1943 trip to Portugal and Spain may have had something to do with Foreign Secretary Anthony Eden using Howard to get a message through to Gen. Franco. (This doesn't strike me as implausible -- the British mobilized more of their cultural talent than Americans did, and the idea of using a charismatic movie star in a diplomatic role isn't implausible. During WWII, top British generals had David Niven, a Sandhurst ex-officer turned Hollywood movie star turned officer, sit in on their meetings with American brass to defuse tensions and to exploit Niven's knowledge of how to charm Americans.)
It's also widely speculated that the Brits decoded the German orders to intercept the commercial airline flight from Lisbon to London, but did nothing in order to preserve the secrecy of Enigma.
So, Scarlet Pimpernel indeed. The Scarlet Pimpernel was Howard's 1934 vehicle about an 18th Century effete English aristocrat who is really a secret agent who sneaks into Revolutionary France and smuggles prisoners out ahead of Robespierre's guillotine. But, who really knows?
So, even though we can't know for sure, we'll say Leslie Howard is a go for third on this list of movie people who died spectacularly in character.