We don't currently expose an API for rendering components. We are planning to expose it as part of the upcoming build series with the web server.
There are no hard dates, but in general, our immediate plans for after the current series of cutting edge builds (which are generally about styles / printing, but also have several important isolated pieces of functionality merged in) center around three general directions: (a) geometry (a rework of some of the key functions for performance and extended functionality + new functions and transforms + sophisticated editing), (b) rasters (an improved data model + new functions and transforms + several new tools), (c) servers (web server is here, but only as a part, there's more).
Please send a feature suggestion regardless of the plans above, any info on what specifically you are after for your use scenarios helps shape what we do. If you prioritize the items with "must have" vs "nice to have, but only after must haves are there", that helps even more.