定语从句中,that能做介词宾语吗?

可以!
当介词后置的时候,介词放在了动词后面,介词不放在that前面完全可以.
但是,紧跟在介词后面指人时用:介词＋whom(不能用who),紧跟在介词后面指物时用:介词＋which(不能用that).而且做宾语的which/whom/that还可以省略.
(1) We’ll go to hear the famous singer (whom/that/who) we have often talked about.
(2) We’ll go to hear the famous singer about whom we have often talked.
欢迎探讨词法语法问题.