I realize that this is a very old thread and apologize for taking this long to comment.
We do have a good sense of the grounds that Titian used. Vasari comments on the grounds of early 16th century Venice (actually, Vasari is an unreliable commentator on technical issues: the erroneous stories about the "discovery of oil paint" and its travel to Italy through de Messina) More importantly technical studies performed on Titian's paintings have shown that he and his contemporaries actually began their paintings by scraping a thin layer of true gesso (gypsum in animal glue) or even flour in glue into the interstices of the canvas weave and covered this with a layer of oil paint. The gesso was really only present in the hollow of the interstice and not a this continuous layer. The thinness of this layer and the superimposed oil (which helped to plasticize the brittle gesso) mitigated the degree of cracking that would definitely have occurred if the gesso was applied in a thicker manner. There are also examples of the application of unpigmented oil to make the gesso ground less absorbent and more pliable. This technique and the use of gesso on fabric supports was replaced by oil grounds in the generation after Titian.
A wonderful and comprehensive chapter covering the grounds used by western painters can be found in the soon to be released Conservation of Easel Paintings edited by Dr. Joyce Hill Stoner
http://www.amazon.com/Conservation-E.../dp/0750681993
Before you run out and emulate the gesso/oil imprimatura ground layering you should realize that it is in no way a panacea and that it was abandoned for good reason. The gesso was probably used simply as a continuation of panel painting preparation. Even with the superimposed oil layer, it is a rather brittle and precarious system. It is likely that the practice was abandoned in preference of oil grounds because of problems associated with cracking, etc.
It is a mistake to assume that the practices of the distant past were always superior because of all of the masterpieces we see in the museums of the world. I think that we often overlook that fact that far more paintings failed or were destroyed so long ago that we are not able to see the devastating results of poor materials and techniques. Also, paintings housed in European collections were often stored in environments that may not have been ideal but which were rather stable or altered slowly throughout the year. There is far less of a problem with a painting being stored in a cold, damp, and dark church for hundreds of years. The problems arise more from environmental fluctuation. This was amply proven when in the early 20th century paintings were brought from these locations to collections in America where the relative humidity and temperature varies widely day after day. This was when transferring and routine lining became so popular and even performed as a prophylactic remedy.
I hope that is of help and interest.