こんなんで行けるかな．．未検証、すみません。
#!ruby
pth="BACKUP.001" #filename
bs=32768 #32KB
str="nothing"
hed=IO.read(pth,bs)
if hed[/LOGNAME/] != nil
ofs=$~.begin(0) #offsetpoint of 1st-char of "LOGNAME"
siz=hed[ofs+11,4].unpack("i*")[0] #size of the object < int
str=IO.read(pth,siz,ofs+15)
end
p str