This gives GUI options for maxuploadtarget and peerbloomfilters, while retaining the user-set values in the Daemon as well (unlike current GUI options). This is done by modifying a bitcoin_rw.conf which is loaded in addition to bitcoin.conf (before, because that’s how our precedence is setup). There’s about 200 LOC for making these updates, but it is all well-tested, and the worst-case scenario doesn’t affect the user-managed bitcoin.conf file.
The ugliest part having to special-case additions to the OptionsModel stuff. It would be nice to avoid that, but I don’t see any way to do so as long as an enum is being used as the interface there.