These are jagwire sport adjusters
"Sport Inline Adjusters Two 50mm lengths of 4mm shift housing with end caps included; not recommended for use with brake systems"
Strong Google-fu DJ, bullet dodged, so what's the alternative for the OP's issue, there must be something suitable out there, although if it were me I'd just use an inline one near the brake levers...
@GreatSince78 started
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These are jagwire sport adjusters
"Sport Inline Adjusters
Two 50mm lengths of 4mm shift housing with end caps included; not recommended for use with brake systems"