I only see まる【虎子】 in daijr/s, koj, and kanjipedia. I don't see it mentioned in any of the entries for まる in other refs.
https://www.kanjipedia.jp/kotoba/0002042400

Web search for "虎子を使っ" only brings up one relevant result, and it's an essay titled "古代・中世のトイレ話"
https://ncode.syosetu.com/n0883fl/2/
  多くの場合伏せた虎の形を模しているので虎子と呼ばれていた。

[arch] seems to be okay

@@ -17 +17 @@
-<misc>&uk;</misc>
+<misc>&arch;</misc>

I'm familiar with おまる but I've never seen it written without the お (I hadn't actually released that the お was an honorific prefix until now). My assumption is that the お-less form is archaic/obsolete.

おまる in the "bedpan" sense is in plenty of modern refs (sankoku, meikyo, JEs, etc.).
Some refs actually have 虎子 read as "おまる" (gg5, sankoku 7e).

GG5 has こじ【虎子】 but in a figurative sense. 「虎穴に入らずんば虎子を得ず」

@@ -24,0 +25 @@
+<xref type="see" seq="2083490">虎穴に入らずんば虎子を得ず</xref>
@@ -25,0 +27 @@
+<gloss g_type="fig">hard-earned reward</gloss>

Aren't both senses archaic? まる and こし aren't in the JEs or smaller kokugos. Koj gives 日葡 as a source for まる.