Can OPENROWSET parameters be built dynamically?
You could in theory do this, but it would require building the update statement with dynamic SQL. Basically, you declare a string variable, build it with metadata from the database, and then execute that string. Erland Sommarskog has an epic post on this: The Curse and Blessings of Dynamic SQL. 

However, that's gonna require a lot of work and a lot of debugging.

Plus, for performance reasons, you don't really wanna fire a new OPENROWSET for every row of a table you're updating. 

Instead, I'd suggest doing a single OPENROWSET call to pull all of the file contents into a temp table, and then doing your update statement from the temp table.