Tricky RewriteCond / LA-U:SCRIPT_FILENAME situation

I would like to determine whether a certain URI resolves to a file and
(if so) what that file is. The following seems to do the trick.

#Look ahead to see how the request resolves
RewriteCond %{LA-U:SCRIPT_FILENAME} ^(.*)$
#If it resolved to a file, assume it's OK
RewriteCond %1 -f

However, this breaks down when I've got URL rewriting going on, but I
don't see how to fix it up. Specifically, given a request,
mydomain.com/path1/path2/path3, if it doesn't resolve to a document,
I'd like apache to determine the deepest subdirectory and then
determine whether there is an index file in that directory.

The first step is to resolve the incoming URI with
LA-U:SCRIPT_FILENAME, which works fine. The next step is to find the
deepest subdirectory with a loop. Works fine, too. It fails on the
next to the last line because LA-U:SCRIPT_FILENAME does not return with
index.php at the end - perhaps someone can spot whether I'm doing
something wrong (same story with REQUEST_FILENAME in place of
SCRIPT_FILENAME):

# Initial test to see whether original request resolves.
# Only entered first time through (controlled by State)
RewriteCond %{IS_SUBREQ} false
RewriteCond %{ENV:State} !.
# What does the filename resolve to?
RewriteCond %{LA-U:SCRIPT_FILENAME} ^(.*)$
# If it resolves to a file, then we're done
RewriteCond %1 -f
RewriteRule . - [E=RewriteDone:%1,L]

# Initialize the loop to find the deepest directory
# Only entered first time through (controlled by Sfx)
RewriteCond %{IS_SUBREQ} false
RewriteCond %{ENV:Sfx} !.
RewriteRule ^(.*)(/[^/]*.)$ - [E=Pfx:$1,E=Sfx:$2,E=State:Part2]

# Loop to find deepest directory - one must exist
# since the root directory exists
RewriteCond %{IS_SUBREQ} false
RewriteCond %{ENV:State} Part2
# If it's a directory, then go on to next section
RewriteCond %{DOCUMENT_ROOT}%{ENV:Pfx}/ !-d
# Otherwise, pop up one level
RewriteCond %{ENV:Pfx} ^(.*)(/[^/]*)$
# By doing a rewriting
RewriteRule ^.+$ %1 [E=Pfx:%1,E=Sfx:%2%{ENV:Sfx},N]

# This section checks for an index file at this deepest directory
RewriteCond %{IS_SUBREQ} false
RewriteCond %{ENV:State} Part2
# This should do the checking, but fails
RewriteCond %{LA-U:REQUEST_FILENAME} ^(.*)$
# Now check whether it resolved to a filename
RewriteCond %1 -f
# And if so append the pathinfo to such fully resolved filename
RewriteRule ^.+ %1%{Env:Sfx} [L]



If more details or explanations are needed, please let me know.
Thanks,
Csaba Gabor from Vienna