As part of a backup operation, I am running the 7zip command to compress a folder into a single .7z file. No problems there as I am using the InVoke-WMIMethod.<
InVoke-WMIMethod
You should be able to do it with a do... while loop that just sleeps until the process is finished.
do { "waiting" start-sleep 10 } while (gwmi -class win32_process -ComputerName $remotehost | Where ProcessName -eq "7za.exe")