The Site24x7 On Premise Poller needs some RAM to run (~ 1 GB) and can monitor other resources without an agent on the network. That may not be the right thing to use to monitor if you have only a few Raspberry pi per network especially because you will need a dedicated VM atleast for the On-Premise Poller Software
Updated: The original poster (ralfaro) wanted to run the Linux agent as a poller itself.
That said we have not thought of making the Linux agent itself behave like a poller. Porting the Linux Agent to run on the Raspberry Pi may take time. However if you could write a script that can pull the important metrics + put it as a cron job to run on the Raspberry Pi, you could use one of the up coming features we are working on. Currently this feature is internally called Custom Monitor. Via that feature you could push any kind of data via a simple REST API call. This could be available in beta in a month's time.
Do let us know if you could run a cron job on each of the Raspberry Pi.
Likewise we would like to understand any technical details that you can share to help us understand how to get this ported. What OS is it running etc.