In our add-on I want to use remote issue links API, but default rendering features are not enough for us. Could I set up custom iframe-based render for a remote issue link?
Yes remote issue links can be looked up from this resource. You are free to render the data available however you like in a web panel. The challenge I think you will run in to is that, for security reasons, little information about the remote link is available in the REST API. You could potentially solve this by having customers install your add-on on each of the instances that are linked, so that way you could make authenticated requests to get details about the linked issues from your add-on.