weird, I came here to ask the same question.

basically I want to do a redirect before the rewrite rule takes effect.

so if my old site was "mysite.com/old/page"

I want to first have that resolve to "mysite.com/new/page"

the way I see it there should be two steps

1) turn the request for "mysite.com/old/page" into a request for "mysite.com/new/page"

2) the htaccess rewrite rules takes "mysite.com/new/page" and performs as usual

how does one do this? surely there must be some sort of .htaccess voodoo to accomplish it?

Just put the rewrite-rules before any SilverStripe rules, but after RewriteBase (if that exists in your setup).
Example:

This will redirect products.asp to /products.. the QSA option tells the rewrite Engine to keep the query string (eg. ?productgroupid=5&language=nl)

wow, I tried this numerous ways yesterday and never got it quite right. I'd have the wrong url with the right content, the right url with the wrong content, totally new weird urls that I didn't intend. anyway, this works, I owe you a beer!

QSA was what I was missing all along.

FYI, here is a useful tweak if you want to direct sub directories

RewriteRule ^old/(.*)$ /new/$1 [R=301,QSA,L]

so mysite.com/old/page now goes to mysite.com/new/page

easy peasy.