I had a look at PDQ's as you mentioned, their Agent is for asset scanning mostly not remote deployment (although they are adding a slight feature). They split their app into two. One for deployments and one for asset scanning where LS does it in one.
The deployment options PDQ have are: Deploy unedited packages from the package library to machines with the PDQ Inventory Agent installed. Even machines outside of your network!
Basically meaning standard installs of Chrome, iTunes, dropbox etc (publically accessible applications), not applications you can package yourself or customise / edit install options. Its basically doing what NiNite does.
There is the list of packages that their agent can install.
https://www.pdq.com/package-library-list/
I think you will struggle to find something that does agent installs on machines off your network that you can specify, customise, etc.
It would be better to look at a VPN client that runs automatically as a service, if they are on your network the VPN doesn't connect but once off it auto connects. Something like FortiClient or Global Protect.