Hi,
If the condition is that there is a one way forest trust:form ABC.com (trusting) to XYZ.com (trusted) .
Based on my experience: when users from XYZ.com logon to workstation in domain ABC.com, the user policies deployed from XYZ can be also applied with one condition :the policy Allow cross-forest user policy and roaming user profiles was enabled in domain ABC.com.
Also did a test : one way trust ,pki.com (trusting), fan.local(trusted) ,user f1.fan.local logon to client 1. pki.com as following:
Deploy a GPO on domain pki.com , and enable the policy : Allow cross-forest user policy and roaming user profiles.
Create a GPO for f1 in fan.local, set a mapping drive policy and hide all the items in desktop policy.
Then when user f1.fan.local logon to client 1. pki.com, all the uer policies (mapping drive policy and hide all the items in desktop policy)applied as following:
Since it is a one way trust, it only works for the situation when users from trusted domain logon to trusting domain.