OK, I may have been hasty in typing my question.
I know that Win98 clients simply log on to the domain and are not "joined" to the domain in the way that a Win2k/WinXP client does. I think my question was whether or not it was possible for a Win98 client to connect to a native mode domain. As it turns out, I was able to test this in a lab environment, and it works without the client extensions. So that part has indeed been solved. Now on to the next part of the question.
We have a multitude of other systems that I can't easily test in a lab. Linux/Unix/Sun systems, AS/400, Macintosh, Maxtor NAS, etc. I am pretty well convinced that these systems are accessed on a per-use basis (with the exception of NAS). For instance, if I have a Win2k client who uses a terminal window on AS/400, they have to authenticate using credentials on that AS/400. Likewise for Mac users. Active Directory plays no role in that interaction. My bosses here in Dilbert-land are concerned that flipping the switch to native mode will prevent users on these systems from connecting to network resources and a variety of other issues. I say no, but I am having trouble getting concrete answers. I am tempted to burn up a $245 Microsoft call on this to cover my behind, but I'd hate to do that if it's not necessary.