I appreciate the dilemma: you understandably want to limit the friction in connecting HN members, and one of the means by which you're considering doing so has historically been abused so people are sensitive to it. I think 'zitterbewegung still has the right idea: it needs to be opt-in.
I don't see an issue with having a user type in an email address from someone's profile. Like the parent says in this case they're basically just using clapchat as their email client. Perhaps they could even set-up an SMTP gateway such that hn users could receive clapchats from emails sent to them @clapchat.io or whatever. Then maybe in the email they send have a tasteful link to also sign-up for clapchat.
Edit: upon thinking about it more, it might be better to randomly generate an address used for each conversation, to prevent a spammer from sending email to someone's @clapchat address
Perhaps it's useful to approach this from the other side.
Say I want to be able to communicate with HN members via email, so I include my email address in my profile. clap.chat makes it easier for clap.chat users to send me email using the email address in my profile, though ideally they want to clap.chat with me. I don't want to use clap.chat. If I don't want to use clap.chat but am getting invitations to use clap.chat, the burden to deal with that is now on me, be it with a filter, or perhaps adding a note to my profile. Perhaps clap.chat includes a record of people who have opted-out.
All of these methods parallel the behavior and handling of spam. Whether or not there's daylight between these types of invites and spam depends on one's perspective.
