can I find a printable ASCII character that never appears in our big file?

If it happens to be true that the data values to be separated are all entirely in the ASCII range, why not just use a single-byte non-ASCII character as the delimiter -- e.g. 0xA0 "non-breaking-space", or if you want it to be visible, 0xA1 "inverted exclamation mark" or 0xB0 "degree mark" or ... (there are several nice candidates).

The situations where you might encounter a non-8-bit-clean process or channel are virtually non-existent these days.

For that matter, is it really entirely mandatory that the character be printable? It seems hard to imagine that making it "look nice" should be an important factor for a 2GB file. Who's going to be looking at it?