how is the filer's name broadcast as an nfs server?
It's not. There is no mechanism, akin to the CIFS browser mechanism, by which an NFS server can announce its existence as an NFS server.
our nfs client is integrated into windows nt explorer. in network neighborhood i see microsoft networks and nfs. when i open up the nfs servers list i see the filer's name.
It's probably doing a hack similar to what the automounter does to implement "/net", such as broadcasting an NFS NULL operation (or a portmapper/rpcbind CALLIT operation for an NFS NULL operation) and collecting the results. Any reply is treated as an indication that the machine in question supports NFS; all it has in the reply is the IP address.