I think I'm going to take a brute-force approach...
I went back and built 702d560 (Mar 21), the first build for 0.10.1.0-SNAPSHOT.
This worked!  No issues with KafkaProducer and initial, post-population offsets were set appropriately.  My send() callback returned no exceptions and reported valid metadata.
Now the fun part... I'm going to try to zero in to find the build, and therefore the change, that broke this functionality. More updates to follow...
Greg

      From: Greg Zoller <[EMAIL PROTECTED]LID>
 To: "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>; Greg Zoller <[EMAIL PROTECTED]>
 Sent: Saturday, April 16, 2016 3:32 PM
 Subject: Re: Producer Bug? Moving form 0.9.0.1 to 0.10.1.0-SNAPSHOT (latest)
   
Not sure if this helps, but I also just tried adding this line immediately before I call my send() code:
p.partitionsFor("lowercaseStrings")
where p is my KafkaProducer and lowercaseStrings is my topic.  
partitionsFor() returned immediately and successfully with expected values for partition information.  Not 100% sure but from the KafkaProducer code it appears this pulls information from metadata, so if that's true I do have a meaningful connection to my server and getting the metadata.

      From: Greg Zoller <[EMAIL PROTECTED]LID>
 To: "[EMAIL PROTECTED]" <[EMAIL PROTECTED]>
 Sent: Saturday, April 16, 2016 3:02 PM
 Subject: Re: Producer Bug? Moving form 0.9.0.1 to 0.10.1.0-SNAPSHOT (latest)
 
Hi, Ismael,
Thank you for your help! 
The only difference in usage between the versions is that I flip my project dependency version from 0.9.0.1 to 0.10.1.0 and clean+test.  Code is identical, including the Docker packaging.  
OK, so I'm looking at the logs and honestly I'm not 100% sure what I'm looking at.  I didn't see anything that looked like an error but that doesn't mean its not there.
I'd be very grateful if you'd be willing to look.  I've attached the compressed logs for just this one failed test and no more, so they're small. (attached)
Greg

      From: Ismael Juma <[EMAIL PROTECTED]>
 To: [EMAIL PROTECTED]
 Sent: Saturday, April 16, 2016 2:30 PM
 Subject: Re: Producer Bug? Moving form 0.9.0.1 to 0.10.1.0-SNAPSHOT (latest)
 
Hi Greg,

I asked if there were any errors in the broker logs because the metadata
request sent by the producer timed out. No messages can be sent if the
producer is unable to retrieve the topic metadata. If the broker has
started without errors, then the producer is unable to reach it for some
reason (network setup or config problem are common reasons).

I know you said this works for 0.9.0.1, but are you sure there are is no
other difference apart from the version? It may be worth trying it without
using Docker.

Ismael
0.10.1.0-SNAPSHOT is the broker too

My procedure is the same as 0.9.0.1.  I do a signed tgz build and then a
local maven install.  I unpack the tgz into a docker just like
spotify/kafka.  I run that as my server broker with ports 9092 and 2181
exposed.

I link my producer code in the gist to my local maven snapshot lib (not in
a Docker)

Same process works for the 0.9 version

Sent from my iPhone