Hi DentArthurDent,
First off, cool nic, say "hi" to Slartibartfast for me would ya? ;)

Getting back to the point. Sorry, but I'm really not into this idea, and here's why:

...The first would be a short field (say 256 characters)...

Surely that is the job of the title of the node, with a bit of thought the general synopsis of the question, meditation etc should be clear. By 'enforcing' readmore tags around longer posts poses a few problems, say for example the post is only a few characters or sentences longer than the "imposed" max length, this will produce another node meaning another Db read (not to mention all the other reads to produce the page) leading to more strain on the server.

The vast majority of nodes (certainly answers) would fall into this category and whilst not a PM developer even I can see that the extra strain of additional Db reads would add yet more delays to an already heavily taxed server. I could be wrong but I don't think I'm too wide of the mark.

So whilst yes, I can see where you are coming from I don't think this is "really" something that could be comfortably implemented.