I see. But still...
Economics and money aside, I wonder if Jefferies had any personal interest in what was essentially his own creation? He could have worked on an Enterprise project, after he retired, in his spare time, solely as a personal project.
As far as screen-accurate work, that'd be up to Jefferies. Any research or details would be up to him. He wouldn't be doing it on a television production payroll, so he'd get final say and wouldn't be limited in any way.
MJ could go for screen-accurate, canon, semi-canon, original design, intended design, or any variation he desired. If fans could accept inaccuracies or variations from FJ, certainly MJ would be allowed similar freedom.
Plus, being Matt Jefferies, many would take his work as gospel, whatever he produced.