Substituting underscores for spaces works, but is still far from ideal...
test
_12345____12___123456
____23__2345______234
The bulletin board software seems to be being quite clever at stripping out what it considers to be redundant spaces and formatting; I also tried enclosing each space character in 'bold' tags, but they were still stripped. I suppose it's attempting to minimise the disk space required to store all these posts...