Welcome on the forums and glad to see your excitement. It appears to happen a lot on people who join this community.<img src="smileys/smiley1.gif" border="0" align="middle">
Thank you for this plugin, I haven't looked in depth yet, but it sure was requested and will be used/useful.
A plugin for cookies is another good idea.
For the question about plugin + PHP combos.
Stuff is, C2 allows for pro developpers as well as amateurs to come together and build applications. The complexity of said applications of course depends on the "level" of the user. Newcomers have some hard time with AJAX and online interactions (webservers, etc...).
This was community lightly-documented with examples in the how do I FAQ in the section AJAX and a tutorial.
Pre-programmed solution makes sense especially aimed for beginners/non web-coding savy persons. "Pros" could use it also or use it as reference to build upon. More documentation on the subject probably would help a lot too.
For the preHTML question, C2 first purpose/general idea is to allow to develop HTML5 games. So yes, exported application only work in the canvas.
Nevertheless, you could consider making a plugin that would modify the DOM (jquery allows for it). Such a plugin would be to recommand to experienced users first hand, users that know what they are doing and where their application might get embedded and so what type of DOM elements might be in said page.
I don't know if it really answer your question, I'm not sure what you mean/have in mind exactly.
Nevertheless there is room for a lot of things.
I'll finish with directing you to the blog article about C2's architecture, a must read in my eyes.
Once again welcome in the community, and congrats on your first plugin, may many more to come.