Recommended way to implement Postfix 'before-queue' SpamAssassin filtering?

None of them! Use just amavisd-new (with SA, without virus scanner!!) as before-queue-filter.

You do it as described here: http://www.postfix.org/SMTPD_PROXY_README.html Even the pros and cons are extensively discussed there.

Before you start you should have a look at http://www.postfix.org/POSTSCREEN_README.html to just drop 70% of all Spam before it gets to amavis.

After that you look at http://www.policyd-weight.org/ to drop another 10%.

When you do before-queue filtering, use a well-suited (tested!) amount of amavis worker threads. And put the tmp directory of amavis into a RAM-disk (described at the amavisd-new website). Without much tuning it then filters 20 mails per second. Tuning will get you more out of it, but do you really have that high traffic (assuming only 20% left)?