This is a late reply and the content my explain why.
There is a set of available classes and methods to fulfill your requirements.
Though you have to be careful as there are side conditions to be considered that require special attention.

You have to wrap these class methods in some routine or class and then plug it into the standard task manager.

From your question I understand 4 different tasks:

• Compact globals in a database   (=force global to make better use of data blocks) ##Class(SYS.Database). Compact(..............)
• Return unused space for a database (=truncate DB) ##class(SYS.Database).ReturnUnusedSpace(........)
• Defragment a database (=make Blocks contiguous)  ##class(SYS.Database).Defragment(........) ##class(SYS.Database.BackgroundDefragment).Start(........)  ;;[not so useful for task manager]
• Compact free space in a database (=really returns disk space) ##class(SYS.Database).FileCompact.(........) ##class(SYS.Database.BackgroundFileCompact).Start(........)  ;;[not so useful for task manager]

The official documetation also has some examples.
My personal favorite is routine DATABASE.int in namespace %SYS
It covers all these functions for interactive use and illustrates also all possible requirements and limits in detail.
The labels you should check are:
COMPACT()
DBTruncat()
DEFRAG()
FileCompact()

This might also explain why there is not just a method out of the box.

Dear @Sylvain Guilbaud 
As this is the oldest unanswered question I found I'll give it a try:
One of the most attractive features of DeepSee is its close link to the Caché (now IRIS) database using 
the DSTIME mechanics for updates.
With an external linked DB you don't have this "built-in" support.
Other DB's have similar features but including them requires some "art work".
And that doesn't seem to happen so often.

On the other hand the question was never expired nor closed.
It might happen that new results (almost 5 years later) show up now as it moves to the front.

J'espère que tu vas bien et en bonne santé!  
Salutations de Vienne,

Robert (toujours actif) 

@Oliver Wilms 
#1) make  sure your ECP Server is willing to accept ECP AplicationServers: 

#2) on your Application Server define your Data Server

%SYS>set Name="MainServer"
%SYS>set Properties("Address")="127.0.0.1" 
%SYS>set Properties("Port")=1972
%SYS>s Status=##Class(Config.ECPServers).Create(Name,.Properties)
%SYS>zw Status
Status=1

see docs: class Config.ECPServers

#3) connect to your Data Server

%SYS>set ecpstate=##class(SYS.ECP).GetServerConnState(Name)
%SYS>zw ecpstate
ecpstate=1      ;; Not Connected
%SYS>set tSC=##class(SYS.ECP).ServerAction(Name,3)   ;; 3 -  Change to Normal
 %SYS>zw tSC
tSC=1
%SYS>set ecpstate3=##CLASS(SYS.ECP).GetServerConnState(Name)
%SYS>ZW ecpstate3
ecpstate3=5   ;; 5 - Normal

see docs: class SYS.ECP

#4) now in SMP:

see my article in dc and the related example on OEX

make sure your / is really just a / and not surrounded by some nonprinting stuff
do a check e.g on https://www.xmlvalidation.com/ 

(ending at line 1 character 183)  could also mean you are missing
<?xml version="1.0" encoding="UTF-8"?>  as first line.

next possible trap: your xml is not encode in UTF-8  (but in some other [windows?]
code after modification with Notepad.exe or similar.

This article may help you.

Effective use of Collection Indexing and Querying Collections through SQL

This might make your task easier

^EDLIST("DIAG", "X123")=" internal health"
^EDLIST("DIAG", "X234")=" External health"
. . . 

So you will need this Function / Method to Update your segment:
The class is just a container for the method

Class ED.Update [abstract] {
/// assumption you have the full Segment already in a string
ClassMethod AddDescription(ByRef segment as %String) as %Boolean
{  set code=$piece(segment,"^")
   if $data(^EDLIST("DIAG",code),value)#10 set $piece(segment,"^",2)=value
        quit $test }
}

now all you have to do

/// ...  get the segment from DG1
    if  ##class(ED.Update).AddDescription(.segment)  {
          /// .....Update the segment in DG1
   }

If the code is not defined you just skip the update