Make up a file name and tell your server about it :)
You can test the UA question pretty easily by using any browser that will let you fake a user-agent. Give the exact text that your robots use, and see if you can get into your site. If no, there is a UA block somewhere. If yes, keep looking for IP.
I really doubt you want to take the RewriteLog approach. If you do, the format is
where-- says Apache--
If the name does not begin with a slash ('/') then it is assumed to be relative to the Server Root. The directive should occur only once per server config.
And once you've done that, you then have to specify a LogLevel. (If you set a log level without naming a file, logs simply vanish into the ether. If you name a file without setting a non-zero log level, no logging gets done. mod_rewrite always has to do things differently from all other mods.)
Apache also says-- with exclamation marks--
Using a high value for Level will slow down your Apache server dramatically! Use the rewriting logfile at a Level greater than 2 only for debugging!
All of this strikes me as a last-resort solution if all you're trying to find out is how the ### you blocked those robots. In fact, this whole section of the docs gives the impression that Apache just isn't all that happy about the RewriteLog idea at all :)