It's possible to do with fuse, but would probably be cleaner with custom tools.
With apt-get-able tools the following kludge is possible:
xmount --in dd --out vdi disk.img mnt
vdfuse -f mnt/disk.vdi
fuseext2 -o "rw" mnt2/Partition1 mnt3
The basic idea is that fuse can be used to separate a full disk image in place into files that point to it's partitions. vdfuse does this, but is a VirtualBox tool and requires a VDI or VMDK file to work. xmount uses fuse to make a raw disk image appear as a VDI file.
Finally once the partition file is available via vdfuse, it can be mounted via an ext2/3/4 tool fuseext2.
It's ugly but it works completely in userspace.
vdfuse should be able to mount a raw image without the help of xmount, but there is a bug which ignores the RAW option.
I tracked down and fixed the bug with a patch here: