Not exactly documented, most people may assume that setting an FTP jail is restricted to absolute paths in Gollem's backend.php .
This is clearly not suitable for servers where users log into virtual sites under different domains. Two styles of jails are then desired:
Trouble is, the site-root location is a variable.
Turns out the backend processing can in fact handle the required shorthand, and one can specify jail root relative user home.
Thus, the following achieves the goal of (2)
'root' => '~/../..',
'home' => '~',
The undocumented part is that setting 'root' requires that 'home' also be specified (Gollem v1).