Run mount -o remount, rw /mnt/sdcard/. This will also work on any external storage, Even an internal mmc card. It won't survive a restart. You could try to modify your vold fstab file. Or add the commands to your .bashrc or profile and reverse it in .bashlogout. You may need to use /storage/sdcard0 or sdcard1 depending what ROM you are running.
Note this will not work on a fuse file system, which unfortunately is the new norm on many new smartphones (EG: Motorola RAZR HD).