"make installworld" to only install new/changed files?

[Rob MacGregor]

I have a strange (maybe) desire to be able to only install the new or
changed files when doing a "make installworld" on FreeBSD (5.2 or later
ideally).  Does anybody know if this is possible?

The reason for asking is that I run a box (shortly to be 2) off of a compact
flash card (since the processor is passively cooled, as is the PSU, this
removes the last moving part, resulting in a truly silent system).  These
devices have a limited life in terms of writes.  The less writing I can do
the better.  There's also the minor issue that sustained writes appear to
lock up the hardware :(

If this is more appropriate in another list, let me know which one and I'll
go pester them instead :)

Thanks

-- 
 Rob MacGregor (BOFH) [PGP key ID 0x1E51BF5A]
        If I cannot bend Heaven, I shall move Hell.
                           -- Publius Vergilius Maro (Virgil).