Metaball imprint.

An animation I’m making is is made up of metaballs, which are combined to look like a dried splash of wax (in this case an envelope seal).
The metaballs scale down to look like the wax has melted and turned into a drip that disappears.

HOw do I put an imprint on the top of them by way of a normal map? I have a letter texture (black and white) that I would like to use.