Hi, Laura. Unfortunately I've nothing new about Peter Flandr(e)au of New Rochelle, the father of Elder Daniel Alpheus Flandrau Sr. My only clue about him is Elder Daniel's letter to his son Daniel, off fighting in the Civil War, which mentions that Peter Flandr(e)au served in Gen. McDoughal's Brigade during the Revolution. What I haven't been able to find are rolls for that brigade, or its history during the conflict, though I'm fairly sure from what I have found that Westchester County residents could have served in it.
Vital records of any kind for late eighteenth century New Rochelle are something of a trick. I made an ill-prepared visit to New Rochelle over the holidays, but managed to miss the Trinity Church office by about an hour, had only about an hour of time in the Westchester County Archives, and found nothing new at the New Rochelle Library (except that the space and staff are very nice). I'm hoping at some point that I might get a look at baptismal records for the latter half of the eighteenth century and manage to tie the different Flandreau lines together and link them back to Jacques Flandreau.
So, no, I'm sorry, nothing new on Peter. Drop me a note if you've anything to share. :-)