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Those last two would not be legal domain names, so Google doesn't interpret them. The "=" would only be legal inside a query string, and "/" would only be legal as part of a (file) pathname. (Colons, slashes, and periods have special meanings when interpreting URLs, and their use is restricted.)
"keyword-keyword" is about as good as you can get.
Jim
In other words, don't do it and don't worry about which browsers, proxy servers, and spiders you're not blocking by not doing it.
Caveat: yes, I know that the Infernal Exploder, in one of its many offenses against standardization, has a way of munging invalid URLs into something else that might have been what was meant. But so what? EVERYONE handles correct URLs correctly--even Google.
If you use an "=" as part of your urls, you are going to have plenty of trouble. I use mod_rewrite on my site as well, however I can see no reason why you need to have an "=" in your urls.
The following url construction would be much beter:
www.servername.com/keyword1_keyword2_3.html
Then your rewrite rule could be:
RewriteRule ^(.*)\_(.*)\_(.*).html www.servername.com/whatever.html?k1=$1&k2=$2&f=$3