Udev rules are never likely to be a "stable interface". Hardware devices and connection mechanisms change over time, therefore new and different information will be produced; the events recognized and reported may change as the kernel's structure evolves; additional function in the applications that interpret udev rules in response to events may demand different udev semantics.
Your suggestion to have a user-oriented application that manipulates udev rules is sensible, though the same factors that force changes in udev semantics will likely mean this application also changes, albeit in a more controlled way from the perspective of the user. There are lots of examples where this sort of application has been written for other facilities. One example: the init scripts in /etc/rc.d were messy to manipulate, so "chkconfig" was written to allow one to see and change a service's configuration without the need to manipulate a raft of symlinks in half a dozen directories. "ntsysv" was written to provide an interactive alternative to the command-line chkconfig program. Some distributions have even fancier, graphical userfaces, such as system-config-services in Fedora. I think SANE is not the right place to develop a udev rules-manipulation application. SANE may assist some other effort with scanner specifics, but we recognize that udev rules is a complicated business: an applicaiton to create and manipulate these rules will be a serious piece of work, will need to address the needs of diverse sets of users and devices, and will require some organization to maintain it as udev changes below and user needs change above it. It is surprising that such an application has not already been written. Maybe it has, and I simply have not heard of it. Perhaps some have tried, but failed because udev information and requirements changed faster than they could adapt their code.