@xdp4: You've probably given up on this by now, but what exactly doesn't work--are the matching hosts in each search not joining together, or is there a count field in both the lookup and the live search that's breaking isnull, or something else? I got a working search very similar to the one you posted in your own answer, so fundamentally this technique is fine; it's likely a problem with your particular data set. Since the join field is IPs, it's probably not an issue of mismatching capitalization, but there might be stray newlines or something that's causing the join to break.
Also, since you're searching all indexes for the past day, it might be as simple as the subsearch timing out before it can return any data. Running a live search over index=* just to see what hosts are reporting in gives me the heebie jeebies. If you're on a moderately recent version of Splunk, try this instead: | tstats count where index=* by host.
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