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.
(section describing userhome-relative paths for root and home deleted)
Turns out the problem was wrongly posed.
Is for example the current domain running horde available as
'hostspec' => $GLOBALS['something...'],
For that matter, is there any quick way to see what global values are available?
'hostspec' => $_SERVER['SERVER_NAME'],
which would howver need to be recast from e.g. 'webmail.example.com' to 'ftp.example.com'
And if ftp-login to explicit IP correctly references the appropriate virtual server configuration for jail path:
'hostspec' => $_SERVER['SERVER_ADDR'],
The last works in my setup at least.