Does janusSEAL for OWA work on Exchange Server 2003 SP2 with security update KB950159?

Yes.
Security update KB950159 for Exchange Server 2003 SP2 does not change the Outlook Web Access files that janusSEAL for OWA hooks into, so if you have an Exchange 2003 SP2 server upon which janusSEAL for OWA 2.1 is working there should be no issue with installing this security update on the server.