Here's the current status on this:
udisks sanely mounts vfat filesystems by using the "showexec" parameter which only makes .exe, .com and .bat files gain the execute bit. Unfortunately, this mount option isn't available for iso9660 filesystems. Adding the execute bit by default for all files in the iso9660 filesystems will result in nautilus querying the user what to do with files, which is something we want to avoid.
Someone needs to implement an equivalent "showexec" mount option for iso9660 filesystems, at which point udisks can be changed to use it by default.