RSS can be used for this, and is being used for this in many situations. It's not the be all and end all as the data to be published as to fit a relatively simple paradigm: a list of items, where each item basically contains enough info to create a nicely formatted hyperlink, that is, a description and a url. There are some extensions to RSS that enrichen the model, but that increases the maintenance on the other website. The power of RSS is that it is a standard way of publishing data that does fit a lot of data, and so it does make it easy for other webservers, web applicaitons, or browser based javascripts to receive and format the data.
But there are a couple of steps that are just not there yet. You write, "much as if they were embedding an image whose source is your OpenACS site". That's a tough one. Embedding an image is a simple as sticking in a url, right? Well to all of our great regret, there is no such thing as a client-side, html include file, and so you need something on the other side, either client browser or web server to read the published data and stick it into the client's browser's document.
So they will almost certainly not be able to just edit their page once to point to the module and then forget about it, not unless their basic web page application technology includes some sort of publishing/subscribe technology. There are however RSS "subscription" scripts out there in perl, javascript, and other stuff, so it may be a good starting point.
