非限制性定语从句用as还是which(涉及as he often was)

When deeply absorbed in work, _____ he often was, he would forget all about eating or sleeping.

划线处除去填 which 外，可以填 as 吗？

When deeply absorbed in work, _____ he often was, he would forget all about eating or sleeping.

这个句子用which和as都可以，关系代词作从句的表语，先行项为deeply obsorbed in work. 用as则兼有比较意义，意为“正如”；用which则没这层意思。