Well, I finally found the way to do it. I'll post the solution as for
anyone in the future having the same trouble.

For each profile, a key is created in
HKCU\Software\Microsoft\Windows Messaging Subsystem\Profiles

Let's say our profile was named "Test profile", so, the key would be:

HKCU\Software\Microsoft\Windows Messaging Subsystem\Profiles\Test
Profile

In there, different subkeys will be created, according to the services
used by the profile, but that is beyond our purpose.

In one of them, and ONLY one of them, there will be a value called
"001e6700", it's a string value containing the path to the default pst
file.

Again, for the sake of an example, lets say in
"HKCU\Software\Microsoft\Windows Messaging Subsystem\Profiles\Test
Profile" I have the following subkeys:

-055380715d92da11950000508baecadf
-065380715d92da11950000508baecadf
-0a0d020000000000c000000000000046
-13dbb0c8aa05101a9bb000aa002fc45a
-295380715d92da11950000508baecadf
-2a5380715d92da11950000508baecadf
-42acdf40ca5b11cdb7ba00aa003cf7f1
-8503020000000000c000000000000046
-9207f3e0a3b11019908b08002b2a56c2
-f85280715d92da11950000508baecadf
-f95280715d92da11950000508baecadf
-fa5280715d92da11950000508baecadf
-fe5280715d92da11950000508baecadf
-ff5280715d92da11950000508baecadf

In one and just one of them there will be a value called "001e6700",
for example, let's suppose it's located at:

HKCU\Software\Microsoft\Windows Messaging Subsystem\Profiles\Test
Profile\065380715d92da11950000508baecadf
String Value: 001e6700
Containing: "C:\Test\test.pst"

***

Analyzing Outlook registry access with Sysinternals Regmon I found
Outlook itself queries on the different subkeys looking for the one
containing the value 001e6700, but just after reading the KB at
http://support.microsoft.com/kb/278940 I realized it was unique.

Hope this helps somebody... at least it helps me, lol.

Thanks everyone for their interest.

Alpha.-