So just for extreme clarity: The simulator examples given here, and the assembly language shown, is only ever for member-specific code, and never for abstract family code? JITs would generate a LLVM-ish intermediate serialized representation?
The notation used in assembler is common across all members. What you can say in that notation is member specific. Thus
addu(b3, b6), addu(b4, 3); is syntactically valid on all members, but only will produce binary on members with two or more adders. JITS (we expect) will produce specializer input notation, or rather a binary form of that, and the same library that the specializer itself uses will turn the JIT code into target binary.