Try mounting a UNIX NFS share or windows samba share. Also you can attach an usb hard drive.
Sorry, i didn't read the edit. About using ps3 free space on Linux, isn't possible as they need different partitions and use different file systems.
As far as i remember, on my 1.50 fw PS3, it gave me the following options for partitioning:
- 60 Gb Ps3
- 10 Gb Other OS / 50 Gb Ps3
- 50 Gb Other OS / 10 Gb Ps3
You can choose any of those layouts, but you cannot access between the partitions. Also you will lose all the data at the hard drive each time you change the partition layout.