No, MBX does not supports glCompressedTexSubImage2D(). Texture compression is a very expensive operation that cannot be done on the fly. Besides, It is not possible just to upload part of a PVRTC texture as all adjacent blocks contribute to the colour.
Dear Iseno-san, Texture compression is done offline by our PVRTC Utility. The API uses glCompressedTexImage2D just to upload the pre-compressed data in memory, so it is as fast as glTexImage2D. The user cannot not modify this data on any way because it is expensive to re-compress so glCompressedTexSubImage2D is not supported. If this kind of functionality is required please use a 16-bit texture format.