Wow this is quite the rabbit hole.
From what I could gather it seems that development of the qemu-guest agent started around mid 2011, at which point Debian Lenny was already superseded by Debian Squeeze.
I could find neither release nor source for any qemu-guest-agent packages from around this time. The Debian package tracker only offers packages for Debian Stretch.
If you want to continue searching you might find something through the use of Google dorking. For example, filter everything before a date before:YYYY-MM-DD`
(e.g. before:2012-01-01
). You can also search the qemu-devel email lists for the tag qemu-ga
which seems to be the tag associated with the guest agent. Apart from that some rather obscure package archives might be your only hope.
It really depends how important the guest agent is for your use case, though. You might reach a point where you would have to exert so much energy for finding this, that it might actually be easier to convince the customer of doing something else.