The RT3200 has the reset button. If you hold it down it goes back to the last full flash and you can start over. I had to twice while setting it up because I put the IP address on the wrong vlan as I turned on full tagging.
You can also have it plugged into a cisco switch and turn tagging on/off as you configure it in case you have a model that doesn't allow you to just reset it like the RT3200 does.
Or you can always set one port to full tagging and one port as an untagged maintenance port, so you can always hook it directly to a laptop. Just make sure to affix a "maintenance port" sticker next to it on the back so pen testers can quickly locate it.