I finally got rspec to install using config.gem (in Rails) with these lines:
config.gem 'rspec', :lib => 'spec'
config.gem 'rspec-rails', :lib => false # Note this hack keeps it from failing on this gem.
...That allowed me to install/unpack them. Which is all well and good. But when I ran my specs, it failed, complaining about an "undefined method `evaluate_value_proc' for class `Spec::Matchers::Change'". Looking around, the claim was that this error was caused by "incompatible versions of rspec and rspec-rails". But that didn't jive with the versions I had installed.
So I commented out my two lines, above. ...At least I had them in vendor/gems, (and my "script/generate rspec" bootstrap command ran fine, earlier).
With the gems commented out, everything worked.
[shrug] Go figure.