In McDowell's 'A Study of Colt Conversions and Other Percussion Revolvers' - he states that no 'factory' conversions took place.
'Private' conversions were done - but on a very limited scale - probably because there were so many Model 1860's and Model 1851's being converted, and thus readily available.
The Dragoon was never a belt revolver - usually being carried in pommel holsters, with the horse bearing their weight - so the demand was probably pretty light during the era - and likely today, as well.
Vaya,
Scouts Out!