Why don't HOST comands appear in the spool?
Yeah spool only captures output from SQLPlus itself. Try this - Before invoking your OS command, turn spooling off. Then pipe the output to the end of the file. Then you can use spool append to start spooling from where you left off again:

spool test.log
select blah from blah;
spool off
!echo %COMPUTERNAME% &gt;&gt; test.log
spool test.log append 
select * from argh;