Actually, after consulting with people who know more about this than I do, it turns out it should be theoretically possible. You would have to link your static library into the MoarVM executable (which could be hairy). But after that, assuming your function is named "frobnicate" and it takes a single 64bit integer and returns a string, it would be a matter of:

use NativeCall;
sub frobnicate(int64 --> Str) is native {*}
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