What can a company do to restrict offsite contract developers from redistributing GPLv2-licensed code modifications?

Question

My company uses a mix of onsite and increasingly offsite contractors for development of websites and online applications.

Our platform uses a mix of open source software and libraries that we've made a number of modifications to over the years. Some of the modified software is licensed GPLv2 without the linking exception. For various reasons, we do not want the source to be made public.

The concern is that if we supply the binaries to our platform for our offsite developers, we are obligated to supply the source code upon request. Additionally, there will be times when we need to distribute the source of our modified libraries. From there, nothing would seem to preclude the contractor from redistributing the work.

The GPL FAQ states:

. . . when the organization transfers copies to other organizations or individuals, that is distribution. In particular, providing copies to contractors for use off-site is distribution.

Furthermore, the GPL states:

You may not impose any further restrictions on the recipients' exercise of the rights granted herein.

The question is: what can be done to restrict offsite contractors from redistributing our modified code?

A note, that I think the GPLv3 addresses this concern with the following clause. So my question is specifically about GPLv2-licensed modified code:

You may convey covered works to others for the sole purpose of having them make modifications exclusively for you, or provide you with facilities for running those works, provided that you comply with the terms of this License in conveying all material for which you do not control copyright.