I don't know if this is any use to you, or if you have already progressed beyond where I got to when messing around with Mercenaries:
http://www.rama.demon.co.uk/Merc.patch
With that patch, using Underfoot, if you spawn a Merc Liason (e.g. #spawn a 1 1 1 1 0 71) and right-click him, it will send a hard-coded response to populate the Merc Merchant window with some Dark Elf Merc data.
If you select one and click 'Hire', it sends another hardcoded response which brings up the 'Manage Mercenary' window with the timer starting to tick down from 15:00.
Some of the OpCodes in there I just made up names for (e.g. OP_MercenaryAssign, OP_MercenaryTimer) as I wasn't 100% sure what they were for.
Anyway, that's as far as I got before I lost interest. I am pretty sure that the OP_MercenaryDataResponse packet is slightly different for SoD, as I originally worked on this using SoD and am sure I had to add a few extra fields when I converted it to work with Underfoot (I think the UF packet struct for that OpCode is identical to Live, at least it was a few months ago).