Not a problem: you can ask the canvas for an arraybuffer to draw on, and pass that to the wasm code. I did this with asm.js in place of wasm (which didn't exist yet) in http://wry.me/hacking/Turing-Drawings/ and you can judge the speed yourself. (As that page describes, there was some overhead from calling from JS to asm.js at least when asm.js first came out. I don't know the current figures.)
asm.js is valid Javascript. It is a valid subset of js that the browser js engines understand, even for the older ones like IE 8. JavaScript has direct access to the DOM, with webassembly, not yet. It may in the future, its still in the proposal phrase (https://github.com/WebAssembly/gc/blob/master/proposals/gc/O...)
So the way to do it today would be WebAssembly -> asm.js/js -> canvas
Good point about the proposal -- that'll be the answer in the end.
asmj.js as JS is irrelevant to the point I was making, because if you access the DOM directly, your code won't verify as the asm.js subset, and so won't run via the asm.js engine.