What version of the gateway were you testing this with? When I tried this on an 8.4 gateway, I was getting incorrect login errors from the Route via FTP(s) assertion. The same policy exported from 8.4 and imported to 9.0 worked with no problems though. There may be a known issue related to this in earlier gateway versions.
Also, if the password is entered into Stored Passwords, you can just select 'Stored Password:' on the Authentication tab of the 'Route via FTP' assertion and then select the Stored Password you want to use. That worked for me in both versions I tested with.