This is a draft of a policy.

/etc/jed.d:
* S-Lang scripts only
* evaluated in lexicographical order on jed startup
* name of files should follow this scheme: <NN><packagename>.sl where
  <NN> is a number of two digits (default: 50) and <packagename> is the
  name of the Debian package that installs this file
* every package _should_ install only one file there. If there are
  _special reasons_ it can be more. Special reason means you have to set
  some flags before the default packages (e.g. NN=20) get loaded and you
  need to load your own modes after the default packages (e.g. NN=70)

Every package should place its S-Lang files for Jed in
/usr/share/jed/<packagename>.

If a package wants to do something after update of the (X)Jed package
it should create an executable file /usr/share/jed/compile/<packagename>
that gets called after each update of jed with the argument "install"
(without quotation marks). The executable can rebuild the precompiled
S-Lang and DFA cache files upon invocation.