At this momenct journalctl couldn't parse corrupted logs. Simple test
test > journalctl -D $(pwd) > out
test > wc -l out
test > for i in $(seq 1 512); do let "out=$i*10"; dd if=/dev/urandom of=system.journal count=1 conv=notrunc seek=$out; done 2>/dev/null
test > journalctl -D $(pwd) > out2
test > wc -l out2
But what should it do? Writing at those offsets basically nukes basis header structures...
It should recognize portions of the file are corrupted and start searching for magic numbers indicating a record from that point. When it finds a candidate magic number, it should check the record's checksum at the known offset from the magic number to verify it actually found a record and not just a spurious magic number. Once it finds an intact record again, it should continue parsing journal records as normal (until the next corrupt region, if any).
ObjectHeader has 6 reserved uint8_t's to use for the magic number and checksum, so this would be a compatible change.
Then you could make a tool (or just modify journalctl) that takes a damaged journal as input and converts all the damaged regions into artificially generated log entries saying something like MESSAGE="Damaged Region #1" with an attached DATA=the original corrupted binary data.
Essentially, journalctl's current behavior of pretending the journal file ends at the first damaged region is unacceptable.
Oh, and because apps can log arbitrary binary data, they could potentially create fake records inside their payloads, so the checksum/hash should be over the ObjectHeader and some random per-journal file value an attacker can't access.