Is there a way to open kmail > new message when clicking an email link on a webpage in Falkon browser?

You might check the default e-mail client in System Settings -> Applications -> Email Client.

I have that set to Use KMail as the preferred email client. I tried hitting a mailto link in firefox and that still opened a new message window from Thunderbird (though I don't think I have the xdg portal stuff properly set up in FF yet), but when I did that in Falkon it opened a new message window from Kmail.
(ubuntu 18.04, Plasma 5.12.7, frameworks 5.47.0, qt 5.9.5, falkon 3.0.0)