Hello Wilco et. al.,
The only solution I can think of is to create a template and define a style
or styles via Format|Styles and Formatting. When you add text to a textbox
just click on the style(s) you want applied.
Another way (which I haven't tested but hope to later today) would be to
have a textbox in your Design Gallery containing all of your text formatting
preferences and then insert them in your publication when needed. However,
the first solution sounds a little bit more flexible.
It seems that to make an Application-wide change to default setting you need
to mess with the Registry. I have always had a fear of the Registry and
simply do not like to go there. There is an add-in listed by MS that
addresses part of your issue:
http://office.microsoft.com/en-us/marketplace/EM011118851033.aspx