Quoting from the Search Manual topic, About real-time searches: "Real-time searches can take advantage of all Splunk search functionality, including advanced functionality like lookups, transactions, and so on. We've also designed search commands that are to be used specifically in conjunction with real-time searches, such as streamstats and rtorder."
Evidently that's not completely true, as you get an error "This command is not supported in a real-time search" when you try to run an
| inputlookup in a real time search. Also, appends don't work, and don't give errors.
index=_internal | stats count by host | append [inputlookup allhosts] | stats max(count) as count by host works as expected on a non-RT search but doesn't show any values in the lookup that are not in the main seach when changed to RT.