OK so I am using a hacky solution to this - basically I use a simple powershell script to change the FROM just before my mails go out.
I run this command:
powershell.exe -noexit -command "&{C:\ReportServer\ChangeFromEmail.ps1 -ConfigFilePath "C:\ReportServer\rsreportserver.config" -FromEmailAddress changetothis@myserver.ca}"
which runs to this script:
Param(
[string]$ConfigFilePath,
[string]$FromEmailAddress
)
Write-Host "Changing FROM Email to $FromEmailAddress"
[xml]$config = gc $ConfigFilePath
$config.SelectSingleNode("//From").InnerText = $FromEmailAddress;
$config.save($ConfigFilePath)
Then I change it back when I'm done using the same script, with different parameter.