也倾向于选择whoever。
我觉得,可以从几部主流词典对whomever的英文释义寻找一下思路:
《柯林斯英语词典》: whomever a formal word for whoever when it is the object of a verb or preposition
《牛津高阶英语词典》: (literary) used instead of 'whoever' as the object of a verb or preposition
《剑桥高阶英语词典》:whoever when used as the object
从英文释义可知,几部词典都认为,使用whomever时,需作动词或介词的宾语。而问题句中的不定式to be一般会解释为主语补语,不定式层级内所缺少的应该是表语,而whomever一般作动词或介词的宾语,所以应该使用whoever。列举3句whoever作表语的实例(摘自柯林斯和朗文词典):
1. Everybody who goes into the region, whoever they are, is at risk of being taken hostage.
2. You've got a message from Tony Gower, whoever he is.
3. This man, whoever he is, must be very clever.
此外,我觉得,也可以把whoever和whomever分别转换为no matter who和no matter whom进行辨别。至于whom的使用场合,也可参考柯林斯词典:whom is used in formal or written English instead of 'who' when it is the object of a verb or preposition,所以,一般可能会这样说:
I don't want to see him,no matter who (*whom) he might be thought to be.
个人理解,仅供参考。