Yesterday I spent some time searching the Web for something on Haydn's
Masses -- which have gripped my attention for a good while, even opening
up a greater receptivity on my part for choral music of all kinds.  I've
loved Haydn's music for some time, but was unaware of these works: some
are as near-perfect instances as I know of Haydn's genius for expessing
unbridled joy within an utterly controlled framework.

In any case, I was looking for some site that could enlighten me about
these Masses.  I came up with very little, but a few biographical texts
I saved did mention Napoleon.  While this might be paltry and of little
help to you, here are two excerpts:

As Haydn's health declined at the age of 77, Napoleon's troops were
invading Vienna. However, Napoleon Bonaparte put military protection
around the composer's house, fully aware of Joseph Haydn's importance.
Despite Napoleon's efforts, the composer stayed true to Austria, and
even wrote its national anthem in 1797.  "Haydn's achievement places
him beside Mozart and Beethoven as one of the great humanists of music"
(Downs 474), and the British thought him to be the greatest composer in
the world (Kerman 493). Mozart's Requiem Mass was performed at Franz
Joseph Haydn's funeral in 1809, where the composer was mourned by a world
who knew his musical worth.

Another source:

While Napoleon invaded Italy on the way to Austria in 1796, Haydn wrote
the Missa in Tempore Belli (Mass in Time of War), in which he used
trumpets and percussion to represent the sounds of war. ...In May of
1809, the nearly incapacitated Haydn was disturbed by the sounds of
Napoleon's army entering Vienna. However, in respect for the ailing
composer, Napoleon ordered a constant guard to stand outside Haydn's
home to protect him from any further bother. Haydn is rumored to have
played the Austrian national anthem every day in protest even as the
enemy guard stood outside. On May 31, 1809, Haydn fell into a coma and
died, but the news of his death was delayed because of Napoleon's
occupation of the city. During the funeral, Michael Haydn's Requiem was
played and at a memorial service on June 15, 1809, they played the Requiem
written by Haydn's dear friend Mozart.

Good luck with your concert: from what I've been able to gather after a
couple of weeks' perusal, the angustiis or Nelson is the gem among these
massterpieces.

Bert Bailey