How can I make it so that any user can chmod in my fuse-mounted ext4-formatted /sdcard?
Chmod will never be supported. That's on purpose according to comments in the source code.

Here's a reference to the FUSE implementation on Github pointing to a comment that says this.

And the original head on Googlesource (a bit harder to navigate than Github).

Even root cannot bypass it:

shell@android: # id     
uid=0(root) gid=0(root)    
shell@android: # touch /mnt/sdcard/x; ls -l /data/media/0/x    
-rw-rw-r-- media_rw     media_rw        0 2013-01-05 17:35 x    
shell@android: # chmod 777 /mnt/sdcard/x; ls -l /data/media/0/x    
-rw-rw-r-- media_rw     media_rw        0 2013-01-05 17:35 x    
shell@android: # chmod 000 /mnt/sdcard/x; ls -l /data/media/0/x    
-rw-rw-r-- media_rw     media_rw        0 2013-01-05 17:35 x


Note that ownership is hardcoded to media_rw:media_rw (below /data/media). PS: /data/media is another story, all the regular permissions apply here, it's ext4 and root is allowed to do whatever it wants.