I think it may be possible to create a group of ‘recent users’ that you could apply the Watching First Post criteria to rather than the trust level.
One way could be to create a custom SQL badge that ran daily based on ‘last seen’. This could be set to auto-revoke for anyone who doesn’t meet the criteria. You can then have an Automation to add the users with that badge to the group (and remove them when they lose it).
Does that sound feasible?