I think i have found all the logic bits in Newest Nodes. But yeah, code search revealed some other modules that also uses this, like the extra "clear" button in the user settings.
Another way around might be to push the "last seen" timestamp about 30 years into the past. That way it's a valid timestamp, but it's just "long in the past" and the automatic limits would kick in in any code i overlooked. And in the couple of bits where logic depends on it (mainly which buttons to show in Newest Nodes), it would be easy to check if the timestamp is a few decades old.
30 years seems a good number, as this shouldn't run into any 32 bit Unix epoch problems this might or might not still exist in the system. And if, in the future, a user logs back in after more than 30 years of inactivity, i'm pretty sure they wouldn't want to start reading where they left off.
perl -e 'use Crypt::Digest::SHA256 qw[sha256_hex]; print substr(sha256_hex("the Answer To Life, The Universe And Everything"), 6, 2), "\n";'