You can add a ini file to the package that would execute the file using the default action similar to what happens in Explorer:
[Avalanche]
[Run]
run=fusion.pkg
Looking at the registry it looks like the .pkg files are extracted by the utility updatebin.exe. The following should work too:
[Avalanche]
[Run]
params=fusion.pkg
run=updatebin.exe
On the fusion download page, it mentions renaming the file to fusion.cab.pkg to automate the install. This may help, but I am not sure how the updatebin.exe operates.