Actually, Tolkien alluded to the Silmarillion's presence in LoTR. JRR pretended that he was just a historian translating into English what Frodo had written (LoTR) and what Bilbo had (Hobbit). However, he claimed that BILBO wrote the Silm. as well!

In the prologue, under Note on the Shire Records, it is written that Bilbo gave to Frodo "three large volumes bound in red leather," concerning the Elder Days, labeled on their backs "Translations from the Elvish, by B.B." It certainly seems that these three volumes were 1) Ainulindal’ and Valaquenta 2) Quenta Silmarillion 3) Akallab’th and Of the Rings of Power and the 3rd Age... Christopher Tolkien even says that he believes that that was what his father meant, in the Foreword to the Book of Lost Tales I.