Question about LVM and volume images on loop

[Paul E Condon <pec@mesanetworks.net>]

I want to allocate several large blank files on HD, 
and make them into virtual hard disk which I mount
using loop. 

I then want to treat them as 'physical volumes' under
LVM, and collect them into a single volume group.

Then, I intend to define a few larger logical volumes
on this 'volume group'. I do this because I want to
store large files on a HD whose hardware interface
limits file sizes to 4Gb, and I want to store larger
files than 4Gb. ( The HD has 500Gb total capacity.)

My problem is that I can't figure out how to execute
pvcreate on my 'image physical volumes'. If I mount
the image, I think I should apply pvcreate to
/dev/loopxx, but that doesn't work, giving a message 
that the volume is mounted. If I unmount the image,
I get (blk0 is my file to be initialized) :

big:/media/WDP-5/blks# pvcreate -f blk0 
  Device blk0 not found (or ignored by filtering).

How can I associate a loop device node with a particular
file, without mounting it? Or how can I get pvcreate
to write whatever it puts on a phisical volume into
a file on disk?  Or how can I mount a loop device without
specifing a fils system type? Or, etc.?

TIA
-- 
Paul E Condon           
pecondon@mesanetworks.net