Not tested, just add the bot to its own userfile with +bot and then .chinfo <botnick> [greet] for all channels, or for channel specific just do .chinfo <botnick> [#chnanel] [greet]. Not sure if eggdrop's default greet system displays global info lines if one is presant and ignores what everys set for the users greet for the given channel. probably only displays it if there isn't one for the given channel *sigh* hope it works anyway.
I'm not sure that a join bind (or any bind for that matter) will be triggered by the bot itself, even with a user record of the bot added to itself. Think you may need to do a raw binding on something like end of receiving names list (366).