6rd is an improvisation on the motive of 6to4 and ISATAP: you have IPv4 prefix which you use as a carrier for the tunneled traffic, the synthesized IPv6 addresses have a part within them that allows to know which IPv4 within your domain to forward the tunneled traffic to.
If the IPv6 is outside of your 6rd domain - then the tunneled traffic is sent to the border relay, which decapsulates it and sends to the rest of the IPv6 internet.
Should be implementable with the firmware upgrade - assuming the memory allows for IPv6 code altogether - and, that the supplier decides to do the update (as opposed to shipping a new model).
A whole lot of el-cheapo boxes are supported by openwrt:
