From LBU 14th ed. § 1114 c

La relative ne peut suivre immédiatement certains pronoms.
Pronoms personnels conjoints : Nous le vîmes QUI AVAIT JETÉ À TERRE SA BELLE CHEMISE BLANCHE (LOTI, Mon frère Yves, X L V I I I ).
— Il est là-bas QUI ARROSE (BOYLESVE, M11′ Cloque, LX).
— Me voici QUI ATTENDS LE JOUR (DORGELÊS, Caravane sans chameaux, p. 2 1 6 ) .
—
En as-tu rencontré ici QUI AIENT LAISSÉ UNE FEMME POURRIR ? (E. et J. DE GONC., Sœur Philomène, XXXIX.)

It seems that often enough this type of relative subordinate (remote antecedent) can be used for conversing. For example sentences constructed on the pattern of the following are not used in conversation.

• Maître Hauchecorne […] pensa que tout était bon à ramasser QUI PEUT SERVIR (MAUPASS.,
  C., Ficelle).

"La" stands for "la boue" (has for antecedent the word "boue"). The antecedent of "qui" (pronom relatif sujet) is "la" and since the antecedent of "la" is "la boue" the referent of "qui" is also "la boue". No, your understanding corresponds to the sense usually given to these words. Moreover, there is not much difference in the basic meaning (in this case) if you use an infinitive clause in place of the relative.

Consider this example;

• Il regarde Hortense travailler./Il regarde Hortense, qui travaille.

By means of the infinitive, the idea communicated is that the person is not watching Hortense as a person to some extent dissociated from what she is doing, but Hortense and how she is doing her work. This is not so if the relative is used because the fact that Hortense is working is an addition (non defining relative clause), and you get the idea that he is watching Hortense without paying too much attention to what she is doing; note that a defining clause here would make no sense.
There is not such an important difference in the sentence under scrutiny. There is no comma in this syntax, but the relative is nevertheless not a defining relative. If it were "on la sent se rouler" there would be some insistence on the sensation caused by the mud wrapping itself around the ankles; instead, the relative makes the wrapping around less central.