well I finally decided to yak shave until I figured this out... here's what I discovered:
It turns out I had 3 (yes three!) different installs of unixODBC on my Mac. I had versions from MacPorts, Anaconda, and Homebrew. I literally didn't know Homebrew had ever been installed on my machine. I blame drunk computing.
I figured this out by searching my box for all instances of odbcinst.ini via the shell
sudo find / -name "odbcinst.ini"
which uncovered /usr/local/Cellar/unixodbc/2.3.0/etc/odbcinst.ini ... in addition to all the ones I was expecting (anaconda and macports). I edited this file adding the Simba SQL drivers as outlined elsewhere, and it worked.
Prior to this I had been editing the odbcinst.ini installed by Macports, obviously to no avail. The reason I was so sure I was using the macports install was that if I ran which isql or which odbcinst it would so files in the opt/local/bin path which is my macports path. Turns out that's not the unixODBC install that R Studio was seeing, however.