Does the anti-microbial affects of hops affect yeast activity and therefore attenuation when dry hopping near the end of primary fermentation?
Haven't seen any scientific stuidies on the topic, but I'd be very surprised if this was the case. For one thing, I don't know that hops affect yeast activity. Secondly, by the end of primary you've got a large, healthy colony of yeast. Adding an ounce or two of hops shouldn't make any difference.
You might want to think about waiting until fermentation is complete before dry hopping. Not because of anti-microbial properties of hops, but because the escaping CO2 will drive off some of the hop aroma that you're hoping to provide.
In most cases dry hopping is or can be done after primary fermentation. So the beer is already fully attenuated prior to dry hops being added.
That said I have added dry hops 3 days into an active ferment and still hit what I predicted to be my target FG. Furthermore, my calculated attenuation has always been as predicted when adding dry hops prior to the end of active fermentation.
No, the anti-microbial properties of hops do NOT affect yeast fermentation at all. Think of all the Double IPA's out there with IBU's in the 80's and higher.
I knew a homebrew shop employee who made a double IPA that had 300 IBUs, including with a ton of late additions and dry hopping (it looked slightly green in the serving line) and it fermented out like a perfectly normal DIPA would when pitched with a standard packet of US-05.
Hops block the activity of Lactobacillus bacteria to some degree, which is probably why they took over the beer-bittering business from the standard gruit mixtures a few hundred years back. Hops do not block Pediococus or Brettanomysees activity however.