I think the only thing you can do is prevent RC from automatically identifying test clocks and test mode signals.
To turn off auto identifcation of test signals:
set_attr dft_identify_test_signals false /
set_attr dft_identify_top_level_test_clocks false /
The defaults are "true".
You will have to manually define test clocks and test mode signals (define_dft test_clock, define_dft test_mode, etc) on the ports that CAN be controlled on the tester. Any internal clocks that cannot be controlled can have the test clock muxed on them using fix_dft_violations.