Well, I guess you can render using raw WebGL calls, but it's not ideal. Construct's renderer has its own batching and state tracking system, so if you make any changes at all to the WebGL state, you will need to save the old state, do any rendering with custom WebGL calls, then restore the old state again. This is necessary to ensure the WebGL state is consistent with what Construct's engine expects it to be (if you change state and leave it, you're pulling the rug out under the engine and will break stuff). Even then, changing loads of WebGL state is slow. So if you go down this route, your next performance bottleneck will be rendering lots of animations simultaneously, as it hammers saving and restoring loads of WebGL state repeatedly during a frame. Additionally if you make raw WebGL calls you bypass the rest of the renderer, which makes it impossible to use things like custom WebGL shader effects, essentially removing the ability to use that powerful (and very useful artistically) feature.
The ideal solution would be to render everything in a plugin Draw() call using IWebGLRenderer. This fully integrates with Construct supporting all features like effects, and works with the existing renderer code and batching system, so you won't need to do things like save and restore masses of WebGL state every draw, and takes advantage of its sophisticated batching system to ensure maximum performance. The renderer is a fairly high-level API and can do many of the things the standard canvas2d can, so it should be feasible - but you might have to change a lot of code, since it works differently.