What's the consensus on mixed content free sites?

[KG Gary]

That sounds acceptable...I think.
I didn't have any problems with these two freesites from early last year, with both basically doing what you are suggesting:

http://www.pynio.com/hairy-porn/frea...ee/index2.html
http://www.pynio.com/bbw/soft-extra-...ee/index2.html

Both freesites use four sets of sponsor content to promote the sponsor's own sites.

I know at least one sponsor whose rules say 2 sets max

I've never seen that rule. Interesting.