How would you recommend to list all REST Services in a namespace?

Here's a method that might get you close to what you want:

ClassMethod GetHostsByAdapter(pProduction As %String, pAdapterName As %String) As %List
{
        Set tPrd = ##class(Ens.Config.Production).%OpenId(pProduction)
        Return:'$ISOBJECT(tPrd) "Production does not exist!"
        Set tItems = tPrd.Items
        Set tItemCnt = tItems.Count()
        Set tHostList = ""
        Set tCnt = 1
        For i=1:1:tItemCnt
        {
            Set tItem = tItems.GetAt(i)
            If $CLASSMETHOD(tItem.ClassName,"%GetParameter","ADAPTER") = pAdapterName
            {
                Set $LIST(tHostList,tCnt) = tItem.Name
                Set tCnt = tCnt + 1
            }
        }
        Return tHostList
}

Call it with:

Set hosts=##class(<classname>).GetHostsByAdapter("<production name>","EnsLib.HTTP.InboundAdapter")

The variable hosts will contain the list (in $LIST form) of business hosts that have the adapter specified as the 2nd argument.