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.

http://baike.baidu.com/view/56536.htm
欢迎探讨词法语法问题。

没有见过
应该不可以

不行，介词后面用which，不用that

看用在什么介词后面啦，有些可用可不能用