The ability to review kanji by RTK lesson has been added back in. The option is available in the "Kanji Review" page. If you pick a lesson, the "from" and "to" fields will be ignored. Not super obvious, but this will do. The usual options also work: shuffle mode, example words and "kanji to keyword" review.

This feature used to be available via "Test" links on the Progress Chart page. However it was confusing users who wondered why the chart did not update after a test. To reduce this confusion, the lesson review mode was moved to the Kanji Review page, so it is clearly separated from the SRS page.

Thanks to sheryll for reminding me about it.