You are searching in vain. There is no subject. The third-person singular form is the result of a semantic-structural necessity, not of agreement between verb and subject.
For verbs with an accusative complement, the werden passive is very intuitive (and readily comparable to English passive constructions):
- Anna liebt [Bernd]Akk.. -> Bernd wird geliebt.
- Christa isst [das Brot]Akk.. -> Das Brot wird gegessen.
This makes sense because in the original sentences, in essence, the subject is doing something with (respect to) the accusative complement -- by using the passive voice and simultaneously turning the accusative complement into the subject, we basically just reverse the way we look at the situation.
Under certain circumstances, verbs without an accusative complement can also be used in werden passive voice. (The key requirement for this to be possible is that the proposition must refer to either an activity or an action.) In these cases, there is nothing that could be "turned" into a subject, and so the distinctive feature of these constructions is the absence of a subject. The passive expresses that the activity/action is "being done/performed", ie it is taking place (there is no direct English translation). As is the case more generally in constructions without a subject, the finite verb here must be a third-person singular form.
Syntactically, the absence of a subject poses a bit of a challenge. In cases like yours where the verb comes last (because of its occurrence in a dass clause), there is no issue, but declarative statements are typically verb-second clauses -- and without a subject, the verb would come first. Therefore, in these cases, an "expletive es" (the terminology varies) is added to the sentence in subject position.
- Dörte und Emil tanzen. -> Es wird getanzt. [= There is dancing happening/People dance.]
- (...) dass Dörte und Emil tanzen (...) > (...) dass getanzt wird (...)
Your example simply poses the extra challenge of a modal construction. Other than that, though, the situation is the same:
Es kann auf eine genauere Darstellung der unterschiedlichen künstlerischen Ansätze nicht verzichtet werden. [= One cannot forego
a thorough exposition of the different artistic approaches.]
(...), dass auf eine genauere Darstellung der unterschiedlichen künstlerischen Ansätze nicht verzichtet werden kann.
For more on such subjectless passive constructions in German, see eg http://clasfaculty.ucdenver.edu/tphillips/grammar/passive_no_subject_info.pdf.
