The only logical place a specific battlemap can be set, if possible at all, is in the startpos files under the header
//========================================
//Border Info
//Attributes of borders between provinces
//and the maps available to each border
//========================================

Let's take a look at one of those lines:
SetBorderInfo:: ID_WESSEX ID_MERCIA 7600 7592 0 LUSH
AT_WESTERN_EUROPEAN INLAND FLAT RIVER

This is the border crossing from Wessex to Mercia. When the game finds a map it will select randomly among all those maps matching the border descriptions:
1)COAST or INLAND
2)Terrain type (FLAT, HILLY, PLAIN, HLL2FLT, etc.)
3) RIVER or NO RIVER
The selected map will then be given correct climate (ex:LUSH) tiles and (ex:WESTERN_EUROPEAN) architecture.

This leaves us with the 3 numbers as unknowns:
7600 7592 0
However, I don't believe they have anything to do with selecting a specific map. To me they look like positional coordinates on the strategy map, like those used when positioning the images of ports and castles. The third number(ex:0) is probably the direction of the image.
Actally I don't even think they are used anymore in VI, because in the VIKING.txt startpos file all those numbers
are set to 0.

To conclude: I don't think it is possible to set a specific battlemap unless there is only one map that matches the given landscape description (INLAND,FLAT,RIVER), but that would mean that every border with the same description will get the same battlemap.