Any way to limit available memory? [5.2.1-R-p5]

It looks like one of my Dell boxes has some memory that is accessible but
shouldn't be.  Sort of peculiar in that it:

    1: Is .2MB in from the upper memory limit, and changes when
	available memory changes
    2: Isn't slot or memory-stick dependent
    3: Can lock the machine up solid when you write certain data
	patterns to it
    4: Doesn't seem to be OS-dependent (Memtest86 ISO, FreeBSD)

Between the data corruption (segmentation violations, mostly), the memory
location (we tend to have problems after the machine has been up and
thrashing for a while) and the symptoms (periodic solid lockups) it looks
like this may be the smoking gun for some of the problems we've been seeing
on that machine.

I'll obviously be pursuing some solutions in the near future (BIOS
upgrades, Dell hardware diagnostics, hard-coding available memory for
operating system, etc).

For the short term, however, is there some way to get the system to think
that we have ~1MB less RAM than it actually tests for?