I'm looking at the "Rules for Interpretation" section (in English) of my 1952 edition of the Liber Usualis, which reads in section IV, paragraph 13:

13. The quilisma : this jagged note [symbol] is always preceded and followed by one or several notes (see examples in the table of neums) ; its value is the same as that of the other notes, but it must always be rendered lightly. The note most immediately before the quilisma should be notably lengthened, and be the most emphasised of the whole group even when preceded by a double note. [Emphasis and italics in the original.]

I suspect that the placing of a dot on the first of a pes, podatus or clivis is a notational convention to ensure that the first note is lengthened as the instruction states. Although in doing a quick survey of puncta or virgi (are those the correct plural forms?) followed by a quilisma, the dot is omitted. I would assume the rule still applies.

I think that the dot in the classic Solesmes rhythmic editions is an attempt to show the weight that is generally in the notes preceding a quilisma. In the St. Gall and Laon manuscripts in the Graduale Triplex the first note before a quilisma is almost always lengthened and in the two cases you mention, a pes and clivis preceding a quilisma, the notes of these are also both lengthened.

Here in the St. Gall notation (below the square note notation) you can see two short horizontal lines before the quilisma (a squiggly line) which indicate lengthened punctum:



And here you can see in St. Gall a clivis (an upside down U) with an episema before the quilisma (a squiggly line). This means that both notes of the clivis are broadened. This is also indicated in Laon with an "a" ("augete", or "augment, broaden") between the two uncinus that precede the quilisma.



Since classic Solesmes asked conventionally for the lengthening of the note before a quilisma it seems that the dotted note seeks to balance the design as it is indicated in the manuscripts.

Interestingly, Gregorian Semiology states that even though the note before the quilisma is typically lenghened the most important note still is the one that follows the quilisma!