Thing is I don't think horizon can do this or should, outside of the horizon interface, the remote apps grab everything from the internal os. so for this
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I can override the icon on the windows RDSH server but that gets lost/overwritten/forgotten during upgrades, changes, new servers etc.
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I'd look at ways to have it persist, something like appvolumes and/or UEM may help, or even group policy may help with persistent the icon.