PLIST fiddling at post-install phase

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

I have a package (cross/avr-libc) that installs man pages whose names conflict 
on case-insensitive file systems (like PRIx8.3 and PRIX8.3).
I think the most elegant way to deal with this is to detect the conflict in the 
post-install phase and then replace the relevant files with some hand-crafted 
merged version from the files directory.
However, how do I adapt the PLIST? It looks easy to detect the conflict by 
something like

PLIST_VARS+= case-sensitive case-insensitive
.if exists(.../pRix8.3) # some funny name that will only match on a 
case-insensitive file system
PLIST.case-insensitive= yes
.else
PLIST.case-sensitive= yes
.endif

and

${PLIST.case-sensitive}.../PRIx8.3
${PLIST.case-sensitive}.../PRIX8.3
${PLIST.case-insensitive}.../Prix8.3

but I guess bmake evaluates the conditional at parse time so it would work for 
make build; make install but not otherwise.
Any suggestion on how to detect a case-insensitive work or dest dir and adapt 
the PLIS accordingly?