Unfortunately we don’t have a concept of variables in InsightIDR yet. However I do remember hearing the idea kicked around a while back.
What you could do in the meantime as an alternative would be to create that long list that you’re referring to and make a custom alert for it but disable the investigation and just apply a label to those logs that contain e.g. “host_name IN [comp1, comp2, comp3]” or something. Then from there when you’re in log search you would just select that label when it appears in the entry view to scope the results down to logs that have those values.
@rboiteau variables are out now in log search, go to settings, log search, and then variables. Input all the values you want to and then simply use that variable in your log search.
Although I don’t know how many values you might have, you could possibly create multiple variables and your query could reference multiple variables in a list.