Can the relative pronoun 'which' have an adjectival phrase as its antecedent?

An American talk show host Jimmy Kimmel was http://www.thedailybeast.com/articles/2017/02/27/jimmy-kimmel-breaks-down-oscars-fiasco-clyde-threw-bonnie-under-the-bus.html as saying:

As “bad” as he was feeling for producers of both films, Kimmel admitted he was also “trying really hard not to laugh.” It was only after Denzel Washington yelled “Barry” from the front row that Kimmel realized he should step away from the microphone and let Moonlight director Barry Jenkins deliver his acceptance speech.

“I did not pull a prank,” Kimmel assured skeptical viewers at home, saying that if he did he would have put a Bed, Bath and Beyond coupon in the envelope instead. “By the way, the producers of La La Land were very gracious, which they did not have to be, on stage and off,” he added.

The underlined portion is a relative clause in a supplementary use, and it seems that 'which', a supplementary relative pronoun, has 'very gracious', an adjectival phrase (AdjP), as its antecedent.
Does it?

If so, can you say in general that a supplementary relative pronoun can have an AdjP as its antecedent?