Current losetup supports creating RO loops: the set_loop has an argument that requests and returns the open mode, and the function also checks if the filesystem the file resides on is mounted read-only. But there is no way to specify the RO mode from the command line. The attached patch adds a new -r option.
Created attachment 139067 [details] patch adding -r option to losetup
FC6 seems like better target for this feature. Thanks for your patch.