I've only tried WF in WSS v3. There isn't any reason why it won't
work in WSS v2 or SPS 2003 but the infrastructure for maintaining state
and responding to task events will be difficult to recreate. While I
think that it can be done, I also think that you will be in for an
uphill battle.
Kinda like trying to security trim the WSS v2 interface. You are months
ahead to simply upgrade to WSS v3 later this year.