[Plugin: W3 Total Cache] HP redirection to wp-content/w3tc/pgcache/_default_.html

Without knowing the rest of your configuration, its likely that if an attempt was made to load “http://www.domain.com/wp-content/w3tc/pgcache/_default_.html” then yes, the rewrite rule would cause that redirect

(I have the inverse rule, but broader – switching *.domain.com to http://www.domain.com – and that caused me similar issues)

no, if w3tc tries to load the cache page from www.* then that will be caught by the .htaccess rule and redirected.
What puzzles me is why w3tc would see the http://www.domain.com address in the first place – my understanding of .htaccess rules (admittedly shaky!) would mean that w3tc wouldn’t see the request as being for http://www.domain.com but for http://domain.com