If you really want to, you might be able to get a Windows resource editor to hack at the cursors themselves, stored in tk.dll (here it is in c:\perl\site\lib\auto\tk\tk.dll). Another option would be to recompile Tk yourself, but that is too much work for me. Of course neither of these options is very portable, but I too did a bit of looking (because it doesn't seem like it should be this hard) and there doesn't appear to be a way to do what you want on the fly under Windows.
One other possibility would be to see how Win32::API or perhaps one of the Win32 GUI toolkits might let you do this. Please do let us know if you get a decent solution!
I'd like to be able to assign to an luser