I've made a userscript that transforms any onyomi reading from hiragana into katakana on Wanikani.
Current version : 2.1.4
Download link : https://greasyfork.org/scripts/747-wanikani-on-yomi-in-katakana
2.1.4 SSL support
2.0.3 Kanji data update
2.0.0 client side lessons
1.9.1 level 48,49,50
1.9 support for kanji listed on vocabulary pages
1.8 level 46+47 and support for the new reviews summary
1.7 added support for kanji listed on radicals'page.
1.6 support for client side reviews
1.4-5 level -> 45
1.3: level 41
1.2: added support for level 39 and 40.
should work with Tampermonkey and ninjakit on Chrome and Greasemonkey