During a recent hardware hacking test I have used a very nice gadget -- NAND Reader sold by Russian company Soft Center. The tool is intended to be used to recover files from damaged thumb drives, but I have repurposed it to read and write the content NAND chips holding embedded OS of the appliance under test.
To use the gadget you have to unsolder the chip, which can be rather tricky, especially if you intend to solder the chip back in place after you have modified the content. You may need to by extra adapter for your chip, unless it fits TSOP-48 adapter which comes by default.
To read/write the chips you need NAND Reader software, available for free if you buy the gadget. Unless your chip is supported by the tool, you have to write the config file, based on the information from the datasheet. If you ask nicely at the forum on vendor's website, you are likely to get help with this.
The gadget and NAND Reader software looks pretty robust and works fine, as long as you stick to the instructions in the manual.
The output is raw binary dump of the chip, with data and spare area (OOB) content intermixed. Actual data can be extracted using nand2bin tool (mtd-utils package on Ubuntu).