Hi @noty,
We don’t have custom icon support at the moment, but if there’s enough interest we can look into this. Please like @noty’s original post above if you’d like this so we can gauge interest.
Technically you can add SVG’s along with other HTML elements within our markdown editor, however support is varied between devices so we don’t official publish this in our documentation. One of the reasons for this is that we ‘sanitise’ the markdown / html once a card is completed, to make them safe for storing and sharing. i.e. tags are stripped. This forum does the same, since you can share content with other users on both platforms.
Nevertheless we will have a look into better supporting inline SVGs as well. May I ask your use case for SVGs? Is it for some cosmetic flair or a particular workflow?