One minor issue with adding hostname column - it will require reverse DNS lookup, which may be slow sometimes. Certainly, in case when it's a problem one can not put this field in select list, but I already see an isssue in this tracker: after execute of 'SELECT * FROM MON$ATTACHMENTS' server sometimes hangs for a long time.
I agree that if there is a chance to make things slower, this feature should not be implemented as an internal field. In this case, Alex UDF proposal looks good, specially if it is included in fb_udf, as he suggested.
Vlad suggestion has one serious plus - it will work not only for TCP/IP, but also for PC/LAN. On the other hand, resolving host name via IP address is client-independent. And I do not think that this approachs exclude one another - we mat add field passed by fbclient and also add an UDF, that will invoke OS name resolver.