I wrote a plugin and right now, I'm working on an installer for it. It turned out to be harder then the plugin itself

I would like to install files in two completely sepperate directories:
1. Typical TARGETDIR (manual, link to site and so on)
2. Plugin directory of a different application (where I install the plugin itself).
The problem is, that I take from registry value of where this main app has been installed( ex: C:\Program Files\MainApp ).
To throw it to this directory is no brainer, however I would like to throw it to directory:
C:\Program Files\MainApp\Plugins.
This is apparently incredibly hard to do, I used Orca to expand the VS.NET possibilites, but without success.
I figured that it would be best if I could somehow change the Directory table in .msi, such that it would always install it to the default location, unfortunately I can't do it.
Is there something I omited?
Is there a good way out of it?
Can it be done with VS.NET installer + Orca ??
Thank you for any help