It turns out, one cannot open unix pipes directly over ssh since Python throws an IOerror. After some wrangling, I ended up doing as follows. Using Python/Paramiko I run these commands on the Solaris server (via SSH):
sleep 10800 > input &
This creates the input pipe and 'sleep' makes sure that the pipe stays open (10800 is 3 hours in seconds). If you don't use sleep, the pipe will close after the first time you pass a batch of commands to it.
The I run my process, by sending this over ssh:
nohup process_name < input > output.txt &
This starts the process, attaches the input pipe to its stdin, and its output to a simple text file output.txt. nohup makes sure the process stays alive if I disconnect the ssh session.
Note: I at first intended to use an output pipe also for the output but since it turns out I cannot open pipes via SSH, there is no advantage using a pipe. I still use a pipe for the input, as this makes sure that my process continues accepting commands as I send them.
The output is easy to read from the output.txt, but for input I send each batch of commands (that I wanted the process to execute). First the Python script creates a text file with the new batch of commands called redir.txt, and then send the contents of that text file into the pipe:
cat redir.txt > input
This redirects the commands first into the input pipe, and then the process reads the commands from the pipe.