Currently --serve-files can only read files that were copied into the trace dir. This makes sense because it was intended to serve various executables and shared libraries to gdb. However, I don't see any reason why we couldn't also write out files that were copied into the trace. Currently in this case you get:

(rr) remote get /tmp/jl_XrKiUH/sys.o.a input2.o.a
Remote I/O error: Invalid argument

Since I wrote this feature this is probably a TODO for myself.