When I took an advanced linguistics class many years ago, we used an
acoustical spectrograph to analyze these kinds of issues. For example, I
worked with a Korean speaker to look at plosive stops and compared them to
the English stops.  I can't help but believe that someone has already done
that with final "d" or the books would have been changed long ago. The
graphic analysis of speech/music that appears in the free program, Audacity,
resembles an acoustical spectrograph; perhaps you could use that to look at
the sounds yourself. Record some speakers of English other than yourself,
making the various final sounds in context of sentences and see what the
analysis shows. 
You talk about using "d" to get the sound of prepositions with initial
vowels "right". What is "right"? I would think that "right" is the way that
native speakers of the language speak the sound. When I try to insert the
"d" sound, I don't get a standard sound. Is this different for speakers of
other varieties of English besides American? I am not sure, but I doubt it. 
Bruce Moon
ESL Teacher
Rio Linda, CA