The rules do not seem to protect the area under your opponent's platform. Objects under the platform can deny your opponent the ability to drive a robot onto their platform, and they can prevent a platform from being balanced. The rules do not seem to prevent this defensive strategy.
Is it allowable to push Game Objects under your opponent's platform?
This would be considered a violation of SG3, via SG10. The v1.0 Game Manual Update included this addition to SG3, which we believe answers your question. If it does not, please feel free to rephrase and re-submit.
<SG3> Platforms are “safe” during the endgame. During the last thirty (30) seconds, Robots may not contact the opposing Alliance’s Platform.
c. Per <SG10>, using a Scoring Object to cause interference with the opposing Alliance’s Platform during the last thirty (30) seconds would be considered a violation of this rule.
A Robot which interferes with gameplay as a result of violating this rule, such as preventing a Platform from becoming Balanced, will result in a Disqualification, whether the interference was Match Affecting or not.