Changing of Exchange ActiveSync User Agent string on HP Palm devices is done by editing /usr/share/dbus-1/system-services/com.palm.eas.service file:
[D-BUS Service]
Name=com.palm.eas
Exec=/usr/bin/mojomail-eas -c {...} Palm Palm/1.0.1
The last two strings stand for Device Type and User Agent.
In some cases changing of Device ID may by required as well which can be achieved using Impostash application (Device Profile -> nduId).
List of observed Device Type and User Agent strings can be obtained from Exchange server reports.