I'm quite interested in hearing more about what you would like to do - I'm having a bit of a hard time following it completely, but I'm quite interested in the idea. maybe an example would help?
Are you asking what an e-mail digest is?
Back in the old days (1988), before the web, Usenet, and its forums, people used e-mailing lists for forums. The problem was one e-mailing list (forum) might generate 30 messages a day. So e-mail list servers (like Majordomo and LISTSERV) would optionally let subscribers join a digest version of their e-mailing list. Basically, the list server combines the day's (or week's) e-mail together into a digest, then sends it out as one big e-mail with a table of contents. There's an RFC specification written describing the digest layout (which could be defined today with an XML style sheet).
When your e-mail client receives such a digest, it displays it in your inbox as a folder. When you click on the folder, it opens and displays a directory of the individual members (e-mails within the digest).
The e-mail list server always assembles the digests in FIFO order. However, for my announcement e-mailing list, I would want the digests assembled in chronological order based on the meeting times of the events listed in the digest. In other words, news releases for each individual event come in randomly. I want them sorted by event meeting date in the outgoing e-mail digest (say based on a sort-key header component).
I think some kind of database that outputs its results according to an XML style sheet (defined by the RFC digest layout) may work the best. You could use The Forms Letter Machine (FLM) as a front end interface to such a database. The problem is you would have to define a different FLM node for each kind of XML tag in the output XML style sheet. In short, the FLM would become an XML file editor.
I have just looked at Help & Manual http://www.ec-software.com/index.html
which DonationCoder is offering a big discount on to registered users. It can do conditional output formatting and has a built-in XML editor. I'm wondering if it can be used to maintain a list of typical events that can be conditionally assembled into a digest defined by an XML style sheet? It would have to have the conditional selection features of the FLM, but work like an XML editor.