Illusion of Gaia had that "feature", too, and it was not THAT annoying (not more than the "ultra slowmo you-got-a-key sound", thats for sure >.>).
The only way to realise this in rpg maker, beside of the mentioned picture-as-character- method, is in mz opinion to prepare many
sound effects containing beep sounds for the following message, e.g. you have a message with 44 characters you play a sound effect containing 44 beeps.
Since one message box may contain 144 characters (as far as i know) you need 144 sound effects.
Or maybe 72 might be enough, no one notices if you use a 44 beep sound at 45 characters.
But anyway, thats completely bullshit, counting the characters of a massage and so on, since i would only use this method if there is really not much conversation in the game (like in IoG
) and i would have MUCH free time to spare.