I haven't tried Nexenta in a long long time, but it seems like it was part of their allocation classes code [1]. The comment at the top of the file says the implementation there doesn't support WBC, but [2] seems to suggest otherwise...

So if you really wanted to, you could try looting code from there, though I believe Nexenta's implementation of that was either orthogonal or significantly different from the one that landed in OpenZFS...

[1] - https://github.com/Nexenta/illumos-nexenta/blob/release-5.3/...

[2] - https://github.com/Nexenta/illumos-nexenta/blob/release-5.3/...