Germany is very twitchy when it comes to human remains. You will have to re-clear security at MUC. I would recommend calling a German Consulate and asking. You don't want Grandma turned over to a funeral director and then have to bail her out at an extreme cost.

Not your exact situation,

From Deutsche Welle:
http://www.dw-world.de/dw/article/0,,1501208,00.html

If, however, an urn is brought back across the border, German law comes into immediate effect, compelling its burial. "It is estimated that some 10,000 urns are brought over the border illegally each year. Anyone who gets caught has to hand the ashes over to the authorities to be interred in a cemetery. But there are no fines and no punishment involved, so people don't mind taking the risk," said Nixdorf.

Any chance of going through London first and doing a hand-off with a Polish cousin? Otherwise you might consider a checked bag as disrespectful as that sounds.