In order to shrink a partition, you need to take in consideration that the filesystem resides on the logical volume. So, firstly you have to shrink the filesystem and after that to shrink the logical Volume. If you do it in reverse, you will probably (for sure) loose your data.
So, clearing that up, here it goes:

Firstly we have to unmount the partition we need to shrink:

core:pool# df -h .
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/datavg-MacPoolLv
                      410G  232G  179G  57% /home/vbox/.VirtualBox/osx/pool
core:pool# [color=green]umount /home/vbox/.VirtualBox/osx/pool[/color]

Then, we shrink the filesystem with resize_reiserfs command:

core:osx# resize_reiserfs -s350G /dev/mapper/datavg-MacPoolLv
resize_reiserfs 3.6.19 (2003 www.namesys.com)

You are running BETA version of reiserfs shrinker.
This version is only for testing or VERY CAREFUL use.
Backup of you data is recommended.

Do you want to continue? [y/N]:y
Processing the tree: 0%....20%....40%....60%....80%....100%                       left 0, 20052 /sec

nodes processed (moved):
int        357 (78),
leaves     59861 (13895),
unfm       60578370 (13712615),
total      60638588 (13726588).

check for used blocks in truncated region

ReiserFS report:
blocksize             4096
block count           91750400 (107479040)
free blocks           31100802 (46828962)
bitmap block count    2800 (3280)

Syncing..done


resize_reiserfs: Resizing finished successfully.

If everything goes well (and it should, it never failed to me), we shrink the logical volume:

core:osx# lvresize -L350G /dev/mapper/datavg-MacPoolLv
  WARNING: Reducing active logical volume to 350.00 GB
  THIS MAY DESTROY YOUR DATA (filesystem etc.)
Do you really want to reduce MacPoolLv? [y/n]: y
  Reducing logical volume MacPoolLv to 350.00 GB
  Logical volume MacPoolLv successfully resized

Finally we mount the partition and see the results:

core:osx# mount -a
core:osx# df -h /home/vbox/.VirtualBox/osx/pool
Filesystem            Size  Used Avail Use% Mounted on
/dev/mapper/datavg-MacPoolLv
                      350G  232G  119G  67% /home/vbox/.VirtualBox/osx/pool