build.sh install vs RO objdir/destdir

[der Mouse <mouse%Rodents.Montreal.QC.CA@localhost>]

There's a 3.0 machine at work, on which I did a build of the 4.0 world
from source, via build.sh's build operation:

===> build.sh command: ./build.sh -D /usr/local/4.0/build/DESTDIR -O 
/usr/local/4.0/build/OBJDIR -U build

This worked fine.  Today, I tried to install it into an alternative
boot disk with

./build.sh -V USETOOLS=never -V 
TOOLDIR=/usr/local/4.0/build/OBJDIR/tooldir.NetBSD-3.0-i386 -D 
/usr/local/4.0/build/DESTDIR -O /usr/local/4.0/build/OBJDIR -U install=/mnt

The reason I'm writing is, this failed, because I had /usr/local/4.0
mounted read-only at the time (I have a log, which I can provide if
anyone wants).  It seems to me that an install of an already-built tree
should be something one can do read-only.

What's the thinking on this?  Am I confused in thinking this is a
reasonable thing to attempt?  Or is this a bug that I should send-pr?
Or is it something that ideally should be fixed but is currently more
work than it's worth?  Or what?

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