你给的问题有矛盾（which不是关系副词,是关系代词）,也不够具体,能给出实际句子吗?
祝你开心如意!When deeply absorbed in work, ____he often was,he would forget all about eating=When deeply absorbed in work, 【as/which】he often was,he would forget all about eating=When 【he was】deeply absorbed in work, 【as/which】he often was,he would forget all about eating最好用as；也可以用which,引导非限制性定语从句，作为插入语。祝你开心如意！