Here's how this could be done as a HN-side feature with zero interaction with archive.is servers:

* Compile a list of domains like nytimes.com that have soft paywalls.

* When a link like https://example.com/ is submitted and its domain is on the paywall list, insert [archive](https://archive.is/timegate/https://example.com/) after it in the title area. Just prefix the timegate part and it's a working link.