The user may want to run external programs in various situations, related to 
the existing functions of the slimserver. Examples:
- When the library is rescanned, full or partial (treat those differently)
- Before a song starts playing

Add the possibility to add the path of external programs and passing of 
parameters using some meta notation, e.g. %d for dírictory path, %f for 
filename. This could either be added in the various server configuration menus, 
or, better, a separate "External Programs" menu may be added. For each 
situation that may be relevant for running external programs, add a list of 
suchs programs and parameter passing (several programs may be executed for a 
certain situation).

One practical example of this would be to apply ReplayGain:

External Programs (menu)
-----------------
The following commands will be executed... (explanation of the feature)

Rescan Music Library/Clear library and rescan everything: <none>
Rescan Music Library/Look for new and changed music: replaygain -r -c %d%f

etc.

I don't know that much about what plugins are capable of. The ability to run 
external programs reduces the need for many dedicated plugins. If the "External 
Programs" may be implemented as a general plugin instead of directly in 
slimserver, this is also fine.