Exactly. If you're looking for some ready-made code, I have some Lua code here: https://bitbucket.org/agraef/pd-smmf/src/master/lua/sclsyx.pd_lua. This also shifts the tuning in the way you described in order to keep the concert pitch at 440 Hz.

This is for a Pd external, but it should be easy to turn the sclsyx:read and sclsyx:convert methods to stand-alone functions to read an octave-based scala tuning and convert it to an MTS 1 or 2 byte scale/octave tuning sysex message.

Albert