As per
claunia/plist-cil#3
I checked and that's what Apple's code does.
tl;dr: it takes a 64-bit number and stores it in 1, 2, 4 or 8 big-endian bytes depending on its value.

So creating an UID object from a number (byte/int/long) should be supported.
Here's my code:
claunia/plist-cil@1979dab