In OgreConfig.h you can find the OGRE_MAX_BLEND_WEIGHTS option. It is set to 4 by default and I never tried setting it to anything else myself, but it is worth a try. (You have to recompile Ogre afterwards of course.)
The Ogre mesh file format itself has no limit for the number of bone assignments per vertex. The binary format simply stores bone assignment as arrays of {unsigned int vertexindex; unsigned short boneindex; float weight;}
On the other hand, seven blend weights seems like overkill, so another way is to leave out the three assignments with the smallest weights and adjust the other four bone assignments.