Upon reading the actual section of the IRC (26 U.S. Code § 6651 - Failure to file tax return or to pay tax), it appears to me that this approach would not work, because the way "Net Amount Due" is defined for use in computing the late filing penalties and late payments, anything paid after the April 15th due date (regardless of whether you received an extension of time to file) are not credited against the net amount of tax due! Thus, IRS is supposed to figure the late filing penalty amount almost the same way as the late payment penalty and late payment interest, by using the amount of additional tax due but not paid as of the April 15th due date as the deficiency amount.
