This was a pretty well established use case in the mid 80s, X9-10 IIRC. You'd have your big mainframe running the X server and a PC implementation of the client providing remote access to the application.
ftp://ftp.ta3.sk/pub/DOS/X.PC/XServers-NonUNIX.txt Provides a good list of commercial X servers, including the one I was thinking of: IBM part #5765-025.
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