In my view at least, Group Notices are similar to blog posts/mailing list emails. Since my opinion of mailing lists is that they are largely made obsolete by modern forum software, I'll stick with the blog posts analogy, and I'll use WordPress as the example engine to use (although this idea does not require WordPress)
Rewriting Group Notices as RSS

Group notices are currently displayed via the Viewer in two forms:

1. Blue box notices
2. Notices tab in the Group Information window

. Nothing about the current way things are presented would need to change, but the back-end would.
Posting a notice

The "Create a Notice" form- when submitted- creates a "blog post" in the new system. The attachment becomes a key/value pair of "asset" and the UUID of said asset. Hypothetically speaking, this could be extended to be a CSV of assets, allowing for entire folders to be attached to a group notice instead of just one item.
Recieving notices via blue box notice

There are two approaches to this:

• Viewer makes request to individual feeds
• Viewer makes request to an aggregated feed

An aggregated feed would reduce the number of http requests involved, but require extra coding on the back-end, whereas individual feeds merely need to check if the Resident is permitted to access the resource.

Individual or aggregated, either way the viewer would:

• Upon login, cURL an RSS feed with an IF-MODIFIED-SINCE value of the timestamp the login occured.
• After intitial request, cURL an RSS feed an IF-MODIFIED-SINCE of the last time the viewer recieved an update.

For those that don't know, when "IF-MODIFIED-SINCE" is used, either the resource is served (in the event it has been modified since the given date), or just a simple header response which indicates that the resource hasn't been updated- resulting in near-zero data transfer.
Viewing Notices via Group Information
Using the current XUI

The attachment column icon is extrapolated from the custom field "asset", the subject is the blog post title, from is the blog post author, and date is the post timestamp.
Using uBrowser, RSS Engine Interface

Since we're using WordPress in this example, a combination of custom themes + plugins would mean the table of group notices would merely be a HTML table, with sortable features granted by some javascript (similar to the sortable tables feature on the SL Wiki). clicking on a row opens the "blog post" in an iframe, or uses Ajax to fetch and display the post content.
Using uBrowser, XSLT

Rather than fetching extra data, if all the pertinent data is presented in the RSS feed, an XSLT transform can be performed on the document, which would then produce the familiar interface.
Reading group notices outside of Second Life

Since the group notices are restricted resources, the Resident's feed-reader would need to support HTTP Authentication. If the Resident name is passed along with the URL to the feed, e.g. http://SignpostMarv%20Martin@my.secondlife.com/group-notices/feed/ , the feed-reader should ask for the SL Password, and store it in it's password manager for subsequent re-use.