Such behavior would inevitably result in an endless stream of bug reports like “DAVdroid reverting changes to local address book. Why?? Error, fix!!1!”
I don’t think this is a good idea, especially because it breaks the whole sync logic and is a dirty hack and creates behavior which can’t be understood by users. For instance, take a read-only file system: it doesn’t allow write access and then magically reverts all written data. Such behavior would cause a lot of inconsistent information, application errors and user muzz. Instead, write operations must not be allowed and result in an error. In my current opinion, no support for read-only address books is better than such a solution.
It could be possible with other account types (which don’t have an edit XML scheme for contacts), but this would require a complete change of the way how DAVdroid works and its sync logic, too…