Last time I tried, the Concertina.net converter treated dominant 7th chords (e.g., G7) as major 7th chords, and it had a limited vocabulary for other types of chords. Nor did they support the extensions which allow one to (re)define chord names, so there was no workaround.

You greatly overestimate the tech-savviness of most users. Anyhow, why should every user have to install and maintain software on their own system when one public site with a web interface would suffice for most folks?