Is there a ballpark figure of the proportion of people who don't get a consecrated burial? Asking for a potential Necromancer.
With the difficulty of storing corpses in non-cold weather, and typical transportation difficulties, surely any hamlet without a local church would bury their dead locally in non-consecrated plots?
And not just isolated hamlets - foresters, bandits, charcoal burners, and presumably a lot more people would be in a similar situation.
Battlefield dead?
It depends on how long christianity taked its roots in that part of the world. Around 1220 nearly every city in Hungary and to the West from the border of HRE use the local church's cemetery. In Hungary there should be a church in everty 10 villages. But ex: in Pomerania or in the depht of the Black Forest a small hamlet probaply offers its deads to the local faerie lord than taking them to the unfamiliar and a bit alien city to have the concacreted burial.
So I would say it depends on the Socio-Economical Development of the community 
Battlefield deads are usually left behind. A good necromancer can use them in large numbers.
Ancient cemeteries (pagan ones, whether tumulus, or roman, or whatnot) and battlefields can prove a good source of ghosts.
That verrry much depends. On a battlefield like that of Bouvines (Sunday 27. July 1214) there are sufficient clergy and sufficient time to have the victor bury the dead - and not doing so would have voided the sacred significance of his victory.
That would surprise me. There is some mention, in RoP:F I think, of funeral trods. To me that suggests that bodies were commonly transported long distances.
Given the significance of the burial to the soul, I imagine people would go out of their way to get a proper burial too.
Quite. Unless they don't care about their souls - like some bandits or cottereaux.
Also some people were inherently denied a proper burial for socio-religious reasons. Any Jew who died away from a Jewish community would not be buried on consecrated ground, the same would apply to Saracens. anyone who died while excommunicated would have the same issue, and anyone whom the local priest decided had died unrepentant might be denied a church burial. In some areas the less religious of clergy might require some form of fee to be buried in consecrated ground.
Not my specialty at all, but I think (at least in northern europe) most people probably got buried in consecrated ground unless excommunicated or entirely unrepentant (or seriously pissing off the priest - which he would be "expected" to be above). Murderers and thieves generally just got buried in worse plots on the northern side of medieval churches (this custom may have originated later though).
At least in my saga we had a few fun plot points around this, lack of unconsecrated bodies for experimentation (soul extraction) etc.
Note that the jews would probably be buried in ground consecrated by rabbi's and thereby under dominion. Any moslem so far away from home (unless in Iberia) would likely not have that luxury and be fair game - but I seem to remember that historically, they were (sometimes/often?) buried in christian cemetaries as by default.
There was still a custom in early 13th century of burying executed criminals on the spot of their execution: not consecrated ground. Also lepers were usually buried were the died by their fellows in misery. Both customs were suspect already in the 1220s, and deprecated - under Franciscan influence - in the 1230s. Soon you would then have a Franciscan friar accompanying the doomed man to his execution.
Are you looking at consecration, burial, corpses, or ghosts?
Raising the walking dead is a fantasy and horror concept more than a medieval one, which does not imply that a mage might not do it.
I would have to check the Divine book, but I would think that all of the Abrahamic traditions have a final blessing, placing the spirit beyond reach.
That reminds me, though; I vaguely remember some rule that bodies were only temporarily immunized from magic, for three days. I could easily be misremembering.
Perhaps your corpse animator might prefer created corpses over organically grown ones. Created bodies vanish, barring ritual, but saves the trouble of preserving and storing them.
At least some of the descriptions we have of ghosts in roman sources, appear to depict walking corpses (with intelligence at least partially intact). At least in the translations I've seen (my latin is very poor, so primary sources are out for me). So maybe not?
I'm now thinking of necromancers flocking to excommunicated towns to note the burial points of corpses so they can later summon their ghosts. Or better yet, arranging the excommunication in the first place (obviously, that's a pretty serious interference with the mundanes, and the consequences of getting caught would be disastrous).
or the entirety of england under the interdict caused by John?
It appears that burial by clergy and in consecrated ground were important enough to generally circumvent the interdict in this respect.
I'm certain that historically all kinds of exceptions existed in practice to interdictions where priests conducted rituals in violation of the interdiction for a variety of reasons of personal ethics. However in RAW the rituals may not be effective if the area is under interdiction and, of far greater significance, the divine aura will drop by 1 point per year, meaning previously consecrated ground may no longer be considered consecrated.
See the same quote from above: the pope himself removed may strictures of the interdict not to lose his many followers among the English clergy.
I came for this response and was not disappointed
according to your link the circumvention regarding burials was unofficial, though the last rights were official, suggesting magi could raise bodies but likely not spirits if the interdiction robbed those rituals of their power.