I managed to reduce the impact of the issue for my customer by importing a list containing the mac address and username into UT.
Searching for the username in UT now works, but finding a server is still difficult sometimes.
Based on the show ARP I retrieved from the ASA firewall (the default gateway) it was peanuts to combine it with a list extracted from their IP management system.
Still the fact that the IP and therefore the hostname field remains empty is bothering the customer.
It really is a shame that such a simple thing as importing an ARP table is not supported from a cisco device.
I hope someone can give me a hint how to work arround this until cisco fixes this gap in the UT.
Cheers,
Michel