They're getting paid, aren't they? Write your interface requirements into your purchasing specifications. 
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This is the way I had always heard it as well, so naturally, I misinterpreted what was actually being said the first time I read a topic in this forum about momentary push buttons. A book I read quite some time ago called Domain Driven Design put a lot of emphasis on the importance of getting everybody's nomenclature on the same page during early development meetings, and things like this are the reason; there's a big difference between a momentary pulse and a sustained signal.
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I would call this a one shot button.
HMI momentary buttons are intended to replicate mechanical PB’s with a NO contact. Depressed closes the contact, releasing opens it. Even those can stick on. Any programmer who is not handling that possibility in the PLC is IMO committing malpractice.
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Getting a tcp connection will not solve all issues for this... Websockets could get you a stable(enough) connection for ping pong isalive stuff.. but its the mouse down itself that can not reliably be tracked in browsers.
You can loose focus of the browser app that it can never know. Which than would think you are still pressing the button and have a connection...
Unless you force the user to hold the mouse button down AND move it around in circles within the button... But good luck with that on touchscreens
But the only reason you would really need this is for some crucial plc control, which you do not want to just be accessable on the whole network... So a real physcal button seems safer
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