could you connect the "insert-text" signal
http://library.gnome.org/devel/gtk/stab ... nsert-text
then have your callback be something like this:
void insert_text_cb(GtkTextBuffer *buf, GtkTextIter *start, gchar *text, gint len, gpointer data)
gtk_text_buffer_get_iter_at_line_offset(buf, &end, gtk_text_iter_get_line(start), gtk_text_iter_get_line_offset(start)+len);
gtk_text_buffer_apply_tag_by_name(buf, "bold_tag", start, &end);
I don't know if this will work or not, mostly because I don't know if the 'start' iter is at the end of the text or the beginning.
I wrote this assuming it was at the beginning, if it's at the end, you'll need to change it to end, and change the old end iter to '-len' from '+len'