When should I add melted chocolate to reduced sugar meringue?

The only ingredients you add to a meringue before the stiff peak stage should be ingredients related to stabilizing the meringue itself - acid and maybe a little salt near the beginning, sugar during the transition from soft to stiff peaks, and starch or other stabilizer at the end of stiff peaks. It's also OK to whip in small quantities of things like extracts or citrus zest near the end.

But if you're trying to actually incorporate something into the meringue, whether it's chocolate chips, melted chocolate, or anything else large, then you always, always want to fold it in after the meringue is stiff, just before you bake it.

Adding anything to the meringue too early or whipping it in too forcefully - especially fat (which chocolate contains a lot of) - stands a very good chance of permanently collapsing it. You incorporate other ingredients at the point when it's most stable and incorporate them very very gently.

If you're only whipping up to the soft peak stage then I guess you'd add the chocolate then, but the result would be very unstable, and most meringue cookies are normally whippied to stiff peaks.

Note that if you're incorporating cocoa powder then you can and should whip it in, rather than folding it in, during the stiff peak stage. Many if not most "chocolate meringue" recipes involve a certain amount of both cocoa powder and chocolate.

As for how it will turn out - it won't be very stable or rise much, the chocolate weighs it down a lot, and expect it to crack significantly. It will still be more like a meringue cookie than a solid block of chocolate, unless you add far too much.

If you want some chocolate flavour but the stability of a pure meringue, just add cocoa powder by itself.