Many PNG compressors allow you to specify the zlib compression level, where 0 = no compression. This will effectively give you an uncompressed image, perhaps with some format overhead.

Your math is a bit off - a 1024x1024 at 32bpp would be 4MB, ignoring overhead.

I've actually done something like this in the past - create PNGs with 0 compression, then compress it with something stronger than Deflate (like LZMA). Because the PNG filtering is still applied, this allows greater compression relative to PNG by itself (or a BMP compressed with LZMA).