Currently, MediaWiki:Discussiontools-replywidget-placeholder-reply is
{{GENDER:|Reply}} to {{BIDI:$1}}
In languages such as Croatian we'd translate this as
Odgovor {{GENDER:$1|suradniku|suradnici|suradniku/suradnici}} $1
In Bosnian they have it as
Odgovor {{GENDER:$1|korisniku|korisnici|osobi}} $1
But {{GENDER:}} does not accept $1, and the "neutral" form is always returned. That can be seen in the reply boxes on my secondary account talk page here. My gender is set to "masc.", and Ponorić is currently "fem." Replies to either one of us show the same incorrect message.
On the other hand,
Odgovor {{GENDER:|suradniku|suradnici|suradniku/suradnici}} $1
which is our current translation on translatewiki (locally, I've changed it to this), the gender in the reply box depends on the respondent's gender: a female responding to a male will see "Reply to Her" instead of "Reply to Him".
TL;DR
Can you make the following work:
{{GENDER:|Reply}} to {{GENDER:$1|him|her|them}} {{BIDI:$1}}