I've built a package of no-ip client and I've tested it in a WRT54GS.
It have a script for init.d/ and other one for configuration.
You can get it here :
http://nets.hopto.org/public/mips/noip2 … mipsel.ipk (compiled)
http://nets.hopto.org/public/mips/noip.tar.gz (package for SDK)