Controlling placement of custodes?

[Matthew Collett]

In the various 'ancient notation' styles, Lilypond automatically places a 
custos at the end of each line, and mostly gets it right.  But when there is a 
refrain or antiphon, a custos may be needed in an unexpected place or at a 
different pitch.  Is there any way to insert a custos manually, or override the 
placement of one?

An example:

Asperges III.pdf
Description: Adobe PDF document

The custos at the end of the third line should be F, not A, since the next note 
to be sung is the initial note of the antiphon, not that of the following 
verse.  Furthermore, it should be placed over the beginning of the 'Asperges' 
incipit, not at the end of the line. There should also be an identical custos 
at the incipit on the last line.

Some ugly hackery with invisible notes can fix the pitch, but I have not 
figured out any way to fix the horizontal alignment, or to obtain a custos on 
the last line.

Best wishes,
Matthew