Specifically, it’s dubious because GPLv3 requires the third party’s modification and/or hosted operation to be exclusively on behalf of / for the client. A judge and/or jury would need to evaluate whether the license is legally valid in that jurisdiction, and whether an attempt to circumvent the spirit of the license through obligating a contracted third-party to use the code would regardless still comply. (For example, QA is permissible use, but what if QA has to be done at scale in a for-profit production environment by the contracted third party, etc.) I expect that a reasonable and demonstrated set of modifications and setting and enforcement milestones would both be essential to a determination in favor of it being permissible use — but of course I cannot guarantee this with certainty, lacking any prior judgments to refer to. (I am not your lawyer, this is not legal advice.)