setup question with open_basedir constraint

OK, I need some expert advice on how to do something like this. I have
an open_basedir restriction in place by my host, and I am hosting a few
sites (that will grow in the future).

What I want is something like this.

/mainsite - application files that I am creating would go here.
/site1 - folder for site 1
/site2 - folder for site 2

I would like to have all my application files underneath /mainsite like
this:

/mainsite
config.php
index.php
...

Then under /site1 and /site2 I would have symlinks (or php files with
include files in them) that would point to the same file in /mainsite

/site1
config.php - pointing to /mainsite/config.php
index.php - pointing to /mainsite/config.php
...

Same for /site2

Problem is I have open_basedir set, so that I can't get to
/mainsite/config.php from /site1/config.php

What are some of my options for setting this up? This would allow me to
essentially have one code base, and then anytime a site (say site1)
wanted a custom config.php file, I could replace the symlink config.php
file with one created just for them, but the rest of the code would
still be from the main code base.

ANyone have any ideas? I have tried to figure out the best way to
accomplish this, and essentially come upon roadblocks.

Would like to hear any ideas or suggestions. Thank you,

JJ

.