Thanks for the @, Greg! FYI, FlowBoost Pro supports Custom Objects via our specialized proxy module (nicknamed "Floxy").
Otherwise, you'd need to copy the CO that has the map of GUID → Name to a Program/Folder token first -- which depending on the rate of change would be totally fine or totally crazy-making 
-- then pass that token to the 'hook.
Either way, you end up with pretty simple stuff:
ownerName = {{ownerGuids}}.find( (itm) => itm.guid == {{lead.ownerGuid}} ).name;
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