And they just added TCP client sockets in Workers. We are just one step step away from being able to serve literally anything on their amazing platform (listener sockets).
Only client sockets are available. So what you can do is build a worker that receives HTTP requests and then uses TCP sockets to fetch data from wherever, returning it over HTTP somehow.
It may depend on the makeup of data or something. They "requested" one of my prior projects go on the enterprise plan after about 50TB, granted the overwhelming majority of transfer was for distributing binary executables so I was in pretty blatant violation of their policy. This was 2015ish, so the limit could also have gone up over time as bandwidth gets cheaper too.