The Practice Direction on McKenzie's has been amended to take account of the recent judgment in N (A Child)  EWHC 2042 (Fam). In short, exceptional circumstances are not a prerequisite for the grant of rights of audience to a McKenzie Friend: although the starting point is that this will not ordinarily be permitted a McKenzie can address the court where an application is made and good reason is shown.
The PD Now includes the following amended section:
Rights of audience and rights to conduct litigation
- Sections 27 and 28 of the Courts and Legal Services Act 1990 (the Act) respectively govern rights of audience and the right to conduct litigation. They provide the court with a discretionary power to grant unqualified persons, including MFs, such rights in relation to particular proceedings.
- While the court should be slow to grant any application under s.27 or s.28 of the Act from a MF, it should be prepared to do so for good reason bearing in mind the general objective set out in section 17(1) and the general principle set out in section 17(3) of the Act and all the circumstances of the case. Such circumstances are likely to vary greatly: see paragraphs 40-42 of the judgment of Munby J. in Re N (A child) (McKenzie Friend: Rights of Audience)EWHC 2042(Fam).
- If the litigant in person wishes the MF to be granted a right of audience or the right to conduct the litigation, an application must be made at the start of the hearing.
I'm not sure that the amended version has yet been published but I posted the link to the April 2008 version here and the section pasted above is the only part to have changed.