yea it will be awesome if we added blocks of HTML and PHP and there are good places for that like the right corner that under (quick access and skin selector) and another places. ::)
[Suggestion] HTML & PHP Blocks
Technically you can already do that, and there are several ways to do it in fact (mods, add-ons, other hacks..), but none of them are properly documented I'm afraid.
Heck, I myself simply edit the HTML in my templates... Because I suck :P