Okay another strange topic here. SpriteFont is great for western languages and good for Asian languages if you're going to use the method described here. The previous method works fine if you know exactly which characters your game is going to use. We got a project here though for an online game, that you have no idea on what the user is going to chat about.
Due to the nature of the game, typing text in Chinese and Japanese is vital. We managed to get the IME support on the input but after that we are stuck on displaying this text on screen. The need for a run-time font rendering method is obvious.

Creating a SpriteFont with a texture holding all the unicode characters is completely out of question (last time we tried we had to abort after three hours of processing). Then pre-selecting the most common characters is also difficult, since the players might decide to write something we didn't include and we can't afford to limit them this way.

One solution we though about is using the classes from System.Drawing to finally produce a texture with the required text. Still this method looks very akward and error-prone since we will have to finetune how this text is going to look on the screen, something that SpriteFont already does smoothly.

So here is the question: Is it possible to create a SpriteFontContent that includes the required characters in run-time? We were thinking about a fixed size texture (a buffer) that would have characters written and taken out in run-time. Still how the whole mix is going to work together? Anybody has any clues on this?

Thank you.

Note: If we could create something like MDX's Font, it would be great.

Thanks for your answer. We got this working using your suggestion of using the System.Drawing components.
Here is an example for people that will try to do this in future. Of course this is supposed to go into a nice and manable class and if you're not as desperate as we are, don't even think about using this because it will guarantee making your game slower. And yes, it won't work in XBOX.

Font font = new Font("Arial Unicode MS", 18, FontStyle.Regular);
Bitmap bitmap = new Bitmap(200, 200);
Graphics g = Graphics.FromImage(bitmap);
g.TextRenderingHint = System.Drawing.Text.TextRenderingHint.AntiAlias;
g.DrawString("Some Text", font, Brushes.Black, 0, 0, StringFormat.GenericDefault);
bitmap.Save("buffer"); //this could be a memory stream

Texture2D texture = Texture2D.FromFile(Device3D, "buffer");

SpriteBatch batch = new SpriteBatch(Device3D);
batch.Begin();
batch.Draw(texture, new Microsoft.Xna.Framework.Rectangle(100, 100, 200, 200),
Microsoft.Xna.Framework.Graphics.Color.White);
batch.End();