Currently it is just silently ignored

rs.reconfig( {_id: 'rs', members: [{_id:0, host:'50.56.91.235'}, {_id:1,host:'50.56.91.254'}, {_id:2, host:'50.56.91.222', hidden:1, priority:0}]})

rs:PRIMARY> rs.isMaster()
{
"setName" : "rs",
"ismaster" : true,
"secondary" : false,
"hosts" : [
"50.56.91.254",
"50.56.91.235"
],
"passives" : [
"50.56.91.222"
],
"maxBsonObjectSize" : 16777216,
"ok" : 1
}

> rs.initiate( {_id: 'rs', members: [{_id:0, host:'50.56.91.235'}, {_id:1,host:'50.56.91.254'}, {_id:2, host:'50.56.91.222', hidden:true, priority:0}]})
{
"info" : "Config now saved locally. Should come online in about a minute.",
"ok" : 1
}
> rs.isMaster()
{
"setName" : "rs",
"ismaster" : true,
"secondary" : false,
"hosts" : [
"50.56.91.254",
"50.56.91.235"
],
"maxBsonObjectSize" : 16777216,
"ok" : 1
}