[susan53]

There's no plural noun in the sentence, so I'm not sure why you would expect a plural verb?

Forget the sentence - you need to concentrate just on the clause "as was custom". The rest is irrelevant. SV agreement is clause based, not sentence based.

custom isn't a noun here, it's an adjective, meaning "usual" "normal" or "customary". The meaning is :

a) He presented an original poem on Harvard's class day. Presenting an original poem on Harvard's class day was custom/usual/normal/customary.
b) He presented an original poem on Harvard's class day. It was custom (etc).

In (a) you can see that the subject of the verb was is the action of presenting - which can be referred to using the pronoun "It" as in (b). The verb is therefore singular.

Alternatively, you can use the subordinating conjunction "as" which is used to describe the nature of something. Elision after subordinating conjunctions isn't unusual. With other subordinating conjunctions, the subject may be elided (omitted) - together the verb BE. Compare :

a) If it's necessary, take these pills.
b) If necessary, take these pills.

ie it's all or nothing : you can include both S and V or omit them both. But you can't include one without the other. You can't say eg :
c) *If is necessary, take these pills

But with "as" the rule seems different. It still works if the subordinate clause comes second :

a) The report discusses employment as it is measured by statisticians, and not as it is experienced by ordinary people.
b) The report discusses employment as measured by statisticians, and not as experienced by ordinary people.
but not :
c) *as is measured... as is experienced

However, when BE is the main verb, the rule changes. Now it often seems obligatory to omit the subject but optional to include the verb. So we now have :
a) *He gave a speech after dinner, as it is usual.
b) He gave a speech after dinner, as usual.
c) He gave a speech after dinner, as is usual.

(c) therefore parallels the construction as it would be if a relative pronoun were used :
He gave a speech after dinner, which is usual.

There is however a slight difference in meaning here. (c) expresses the idea "as is usually done" while (for me at least) (b) indicates "as is usual for him". That's why, I think, in your sentence, the omission of was would sound odd - custom always means "as is usually done" and therefore would be less likely to occur in the (b)- type structure because of meaning - not because of the grammar of as.

NB : you'll easily think of sentences with "as" which contradict what I've said, but be careful - as has many other meanings (eg when, the same as and because) and the rules certainly don't hold for these other meanings. But if anyone can think of examples with the meaning "the nature of something" that contradict them, I'd be interested. I can't claim to fully understand what's going on here. It also seems to depend on clause order and the type of subject involved - it seems very complex.