Why does Unicode need to represent a half-written Japanese word? If it's half-written, you're still in the process of writing it, and this is entirely the domain of your text input system.
Which is to say, there is absolutely no need for the text input system to represent all stages of input as Unicode. It is free to represent the input however it chooses to do so, and only produce Unicode when each written unit is "committed", so to speak. To demonstrate why this is true, take the example of a handwriting recognition input system. It's obviously impossible to represent a half-written character in Unicode. It's a drawing! When the text input system is confident it knows what character is being drawn, then it can convert that into text and "commit" it to the document (or the text field, or whatever you're typing in).
But there's nothing special about drawing. You can have fancy text input systems with a keyboard that have intermediate input stages that represent half-written glyphs/words. In fact, that's basically what the OS X text input system does. I'm not a Japanese speaker, so I actually don't know whether all the intermediate forms that text input in the Japanese input systems (there's multiple of them) go through have Unicode representations, but the text input system certainly has a distinction between text that is being authored and text that has been "committed" to the document (which is to say, glyphs that are in their final form and will not be changed by subsequent typed letters). And I'm pretty sure the software that you're typing in doesn't know about the typed characters until it's "committed".
Edit: In fact, you can even see this input system at work in the US English keyboard. In OS X, with the US English keyboard, if you type Option-E, it draws the ACCUTE ACCENT glyph (´) with a yellow background. This is a transitional input form, because it's waiting for the user to type another character. If the user types a vowel, it produces the appropriate combined character (e.g. if the user types "e" it produces "é"). If the user types something else, it produces U+00B4 ACCUTE ACCENT followed by the typed character.
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