Possible to use Piston to manage Mephisto installation while still using svn:externals for vendor/rails?

Hello,

Is it possible to use Piston to manage a Mephisto (or any other app)
installation while still using svn:externals to manage vendor/rails? I
do not
want to commit an entire copy of Rails to my repository just to use
Piston to
manage Mephisto -- I have made no local changes to Rails, only to
Mephisto.
Though it appears simple enough, my problem with this configuration is
that
when running "piston update", I receive the following notice:

Skipping '.': pending updates -- run "svn update ."
X vendor/rails

Performing status on external item at 'vendor/rails'

Even after running "svn update", I receive the above notice. Is it
possible to
continue using svn:externals to manage vendor/rails while using Piston
to
manage and update the root Mephisto directory?

Thanks,

Jeff