Yes, the check could be placed somewhere other than jcheck.
jcheck was recommended because the optimal place to fix incorrect legal notices is before a changeset is checked in, just like it's done for other source file formatting checks.
Changeset comments and source files do not contain tabs, carriage returns, or trailing spaces.
It's undesirable to perform the check post commit since a separate bug to fix the legal notice error would be needed.
Pre commit check outside of jcheck is fine, as long as it's always performed automatically.
Any non jcheck solution should to be accessible outside of Oracle, as commits happen there as well.