I actually had a look at this a while ago. Unfortunately the compiler source is a right mess of cross-references all over the place so it's a hard nut to crack without getting lost along the way.
From what I've been able to determine the compiler puts placeholder labels in the assembly for function calls that are actually resolved at runtime by your compiled program itself rather than at link-time. This means that all function code is essentially put in a block somewhere in the executable and as such it should (theoretically) be relatively trivial to just put precompiled code in there. Problems will however arise if one such precompiled function refers to another function, that may then have a different placeholder identifier at the time it got precompiled vs. when the executable got built.