It seems a recipient for a new message via Mail has two properties; name and address. When using the 'send mail' script step, FileMaker is setting the address property only, and won't do anything with the name. It seems the name property can be set with a script by using AppleScript, and just recreating the entire 'send mail via client'.
FYI, on Windows (7) FM15 can make a mail with fully specified To: string (name<address>) with Thunderbird.
So it looks problem of Mail.