"As is customary" vs. "as it is customary"

Actually, it's not possible to answer this question in its present form because there is no context.

As is customary is used in one type of sentence,

and

As it is customary is used in two other types of sentences.

Without full sentences, there's no telling what the second phrase means. Let me illustrate in these sentences about military funerals:

As is customary, someone played Taps, and seven enlisted men gave the dead soldier a twenty-one gun salute. This is merely a description of what happened.

versus

As it is customary, someone played Taps, and seven enlisted men gave the dead soldier a twenty-one gun salute. This explains why what happened happened and gives a reason for what happened; "as" means "because" in this sentence but not in the previous sentence.

versus

As it is customary to do, someone played Taps, and seven enlisted men gave the dead soldier a twenty-one gun salute. This explains why what happened happened (which may be construed as a reason, I suppose), but "as" doesn't mean "because" in this sentence. It is an unelided version of the first sentence.

versus

Because it is customary to do this, someone played Taps, and seven enlisted men gave the dead soldier a twenty-one gun salute. This explains why what happened happened and gives a reason for what happened; "as" means "because" in this sentence. It is an unelided version of the third sentence.

All four are grammatically and semantically correct, but the second sentence is different because it uses this ambiguous little two-letter word that so many native speakers of English don't mind using to confuse readers and listeners. This should demonstrate quite clearly that there is no "dummy it" needed and that the first is not an elided version of the second.

Quite simply, 'as is customary' adds context to an action that will be occurring regardless of the accompanying statement, whereas 'as it is customary' explains why the action is happening in the first place.

Here's an example of the first:

As is customary, Aunt Nina gifted the kids wristwatches for Christmas.

This means:

As she is known to do, Aunt Nina gifted the kids wristwatches for Christmas.

We don't know why Aunt Nina's gifting the kids these wristwatches, we just know that she usually does.

Now, for the second:

As it is customary, Aunt Nina gifted the kids wristwatches for Christmas.

This means:

Because she is known to do it, Aunt Nina gifted the kids wristwatches for Christmas.

Here, we do know why Aunt Nina's gifting the kids wristwatches. It's because she usually does and she's expected to again. Hope this makes sense.