A post on the HL2world Forums outlines the fairly simple method for getting a Half-Life 2 deathmatch server (as opposed to Counter-Strike Source) running (thanks sauron). This is unsupported and certain aspects of this are a bit wonky, for instance, there doesn't seem to be a Gordon Freeman model for this purpose. Here is the sequence of console commands they've worked out:

    net_start
    sv_lan 0
    deathmatch 1
    maxplayers (whatever you want)
    map (mapname)
    restart

It will add you to the master server and it works.