@viro, you appear to be saying that a kernel module will need MORE definitions than the "subset of [them] that covers syscall-related constants and structs".
1) If that is so, does this mean you consider constants and structs potentially copyrightable?
2.1) If the answer to 1) is yes, please consider that Bionic contains approximately 750 files containing a total of approximately 2.5 megabytes of constants and structs (plus other material, but the bulk of it is constants and structs). At the start of each such file, Google claims that this is "no copyrightable material" and threw out any copyright and GPL notices found in the raw headers.
2.2) If the answer to 2) is no, of if you wish to add something anyway, please specify what your derivative work theory for a loadable kernel module would be.