Currently it can be only logged into nxlog.log and via im_internal. There is no built-in support to write into the windows eventlog (i.e. there is no output equivalent for im_msvistalog), though this is on the roadmap.
You could do this with om_exec though by setting up a route with im_internal => om_exec and have om_exec invoke a script that writes into the eventlog. eventcreate or the Write-Eventlog powershell cmdlet can do this.