I think that may have done it?
I initially used the &contexts parameter and just entered the subdomain context, but that did not work.
I then axed the entire subdomain context and started it from scratch. I then redid all the steps outlined here:
http://rtfm.modx.com/display/revolution20/Making+Member-Only+Pages for member pages then made sure to include the contexts paremeter in the snippet with both the web and my subdomain context - and everything is working now.
For whatever reason, it looks like I needed both contexts specified in the contexts parameter to make it work. And now after the fact, this comment seem all to obvious…
http://rtfm.modx.com/display/ADDON/Login.Login?focusedCommentId=44236883#comment-44236883
Thanks all for your help.