The runScript function can be used in expression tags, but the poll rate doesn't work exactly the same as in an expression binding. All Tags have a Scan Class that dictates the minimum amount of time between each evaluation. The runScript poll rate only polls up to the rate of the scan class set on the tag.
For example, if an Expression Tag is configured with runScript to run at a poll rate of 60 seconds and is using the "default" (1 second) scan class, the Tag's Expression will still execute every 1 second. So a scan class rate of 60 seconds will be necessary for a runScript expression to poll at any rate between 0 and 60 seconds.