>    
>    1. Is unionfs stable enough for this?
> 
> well, probably.  it seems to work OK if you don't access it at multiple
> concurrent times (eg, 'make -j6 build' :).
hm, I just do the following on a regular basis an my ss20:

    rsync -Cav <some-source-box>:/usr/src/ /usr/src
    mount -t union /usr/lsrc /usr/src
    ...maybe config new kernel...
    cd /usr/src/sys/arch/sparc/compile/<kernel>
    make -j7

works like a charm...

as others have stated allready, you should cd(1) /<somewhere> before
reboot(8)ing...

just my tow cents,

-ks