On a normal blog (i.e. Wordpress), widgets are easy to add to the sidebar, footbar or even to the post itself. The editors from Steem interfaces filter HTML code to the bone. Javascript is n't an option for a widget code either. There are security reasons for filtering HTML and javascript out drastically. But you lose in flexibility if you want to do something like you proposed.
dClick most likely searched for a way to extend their service to the entire ecosystem. Fundition may have explored it for what you're proposing, an integration of a donation per project widget within blog posts or other interfaces. I would be the first to applaud if or when such a possibility would be open, without having serious security concerns. Maybe steemit.com can do it, since they split the wallet from the social app?
Creeping through the github I saw something about whitelisting iFrames from certain projects. I wonder if it could be done by allowing the code entries from "trusted sources".
I haven't seen that, I mean other than, for instance, the iframe for Youtube videos, on interfaces where they play inside the post.
Allowing more types of iframes from other trusted sources will be the way to do it, if we want to explore the widgets territory.