Now that he's no longer in office, the Reagans might have the courage to "just say no", but, recalling the funeral of Nixon (the only one for an American president to take place since LBJ in early 1973 when the acrimony between the two parties wasn't what it is now), I don't think it would be according to protocol to turn away a former president wishing to attend. At best, we can either hope Clinton wouldn't want to go or would not desire to give a speech.
But, honestly, could Clinton resist the temptation to be seen as relevant again by the international press?
No. Clinton would drone on and on and likely try to dress up in the mantle of Reagan's greatness, saying what an inspiration he was and how Clinton built on Reagan's work, and so on and so forth.
That's why I'm hoping God will finally get angry and let fly with the lightning bolts.
Regards, Ivan