Note: This is just a speculation
Possibly, iOS version may have a different extension mechanism. Extensions could have two variants - one for Android as usual (written in Java using Android APIs) and the other one for iOS (written directly in Scheme/YAIL probably [not sure about access to iOS APIs but it could interoperate with native APIs via the interpreter]).
When you connect the companion to the builder, the companion would execute only the variant which it supports.
PS: There could be another variant for the builder itself, which would show the preview of the visible component. This mechanism would help me to achieve my GSoC proposal more realistically.
@ewpatton, tagging you, hoping that you read this. How's the idea? Is it close enough to what you have planned?