I will have to look the next time I go back to a certain site how we did this... we simply used a Grandstream HandyTone device, not sure what model as we have used a lot over the years, and an ONS MOH adapter from an SX-200 ICP (P/N 50006162). The adapter has a 600 ohm short on the line side that plugs into the Grandstream and holds the port off-hook and the music hooks to the other side, somehow we had a bunch of these on our shelf because the SX-200ICP kit would come with one and the sales people would always order an extra one (something weird in the configurator I guess).
I don't remember exactly what we did, was a couple years ago, but I vaguely remember the sales person telling the customer it would take something like 4 hours and we were done in like 20 minutes, so it must have been pretty simple.