Did some further tests, and this is not allowed via the vSPhere CLient nor the Web Client either.
That it works from the DCUI, seems to be because the API method doesn't seem to do any checking.
In fact, this is accepted, but I'm not convinced it is a valid configuration, and if it will do what you assume it to do.
$esxName = 'MyEsx'
$esx = Get-VMHost -Name $esxName
$netSys = Get-View -Id $esx.ExtensionData.ConfigManager.NetworkSystem
$spec = New-Object VMware.Vim.HostDnsConfig
$spec.SearchDomain = 'full.domain.com','*test'
$spec.Dhcp = $esx.ExtensionData.Config.Network.DnsConfig.Dhcp
$spec.DomainName = $esx.ExtensionData.Config.Network.DnsConfig.DomainName
$spec.HostName = $esx.ExtensionData.Config.Network.DnsConfig.Hostname
$netSys.UpdateDnsConfig($spec)