Controlling placement of custodes?

[Bill Mooney]

You wrote:-
+++
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:
 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
+++++++++++

Greetings Matthew,
I cannot help with the detail of how to achieve the custos movement you 
ask about...

However, my understanding of the Asperges is that the Antiphon is 
repeated only after the end of the Gloria Patri... See p11 in Liber 
Usualis ed. Benedictines of Solesmes, Desclee & Co. 1959
(not within the chant as shown in your pdf file)

Regards
Bill