I believe the ZIL resides on SLOG first and data VDEVs second, so Very Bad Things would happen at 100% data VDEV capacity even if there is technically free space on the sVDEV. The SPA would throw the error trying to write into the ZIL first before it got a chance to write to sVDEV. Asynchronous writes that fit entirely into sVDEV might succeed in that scenario but the updates to meta/uberblock would probably still make it go “nope” on the final txg sync though.