Mounting ISO files on Azure is as simple as calling the existing cmdlet Mount-LabIsoImage. The only difference is, that the ISO image file path lies on Azure. To find out which images are accessible in your AutomatedLab storage account you can do the following:
Login-AzureRmAccount (Get-LabAzureLabSourcesContent -RegexFilter \.iso).FullName

When mounting the ISO file, be sure to specify -PassThru to be able to use the drive letter that was used to mount the ISO in later commands:

$mountedVolume = Mount-LabIsoImage -IsoPath https://some/azure/path.iso -ComputerName DC1 -PassThru

Invoke-LabCommand DC1 -ScriptBlock {
param
(
    $DriveLetter
)

    Start-Process (Join-Path $DriveLetter "Path\To\My\Setup.exe")
} -ArgumentList $mountedVolume.DriveLetter