How can I query a remote server using a local function?
Seems (based on the name, anyway) like a pretty basic scalar UDF that just massages strings, won't be subject to a lot of changes over time, and doesn't rely on any data. So, I suggest you make a copy of the function on the source server, and fight whatever rule is preventing you from doing so (it shouldn't be hard if you can justify the use of the function, and describe what kind of hack you're trying to use as a workaround to the rule). Note that the function doesn't have to live in the same database, it just needs to be on the same server.

(Even if the function does rely on data, and it's clear in your case it doesn't, the local copy of the function could reference the data in the other server, using 4-part name(s), without referencing any functions.)

DRY is not always worth pursuing.