The key phrase is, system partition.
What I don't want to do: use full disk (device) encryption. That seems to be what ubiquity is able to do on install, and what many of the online guides describe.
What I want to do. I have a device with multiple partitions (2-3 with linux, plus windows). For ONE of those linux installs, I'd like to fully encrypt that partition, and have it behave just like it behaves for full disk encyption. That is, during boot of that linux partition, it asks the user for the decryption password, and continues with boot.
I'm willing to do a fresh install to make this happen.
Thanks!