Hi,
lwIP for Altera is old and not recommended for new design (on their opinions).
I had made many "deep"(google cache, white papaers...) researches too. Sorry, I can't help you. I abandoned my researches and my project is suspended (objectif changed).
You should consider InterNiche Stack which, in my opinion, "can" be ported to FreeRTOS.
lwip : advantages : free, open source, ported to FreeRTOS....
inconvenients : not stable, hard to implement (site is very confused), Even if there are documents, it is not easy to use. not many examples with MAC/phy.
i suggest to look at billa's post. he posted a driver...... (thanks to billa). interniche stack : advantages : good support, used by Altera (with ucOS II), used by a few users in "superloop mode", support usual MAC/PHY. I think it can be ported to any OS (see sources)
inconvenients : not free, hard to implement (i mean not obvious, heavy modifications, huge for 1 only designer). For me, too expensive for 100 products for just sending text by Ethernet.
I stayed interested if someone have succeeded in using LwIP with FreeRTOS with NIOS II on Altera devices.
Other suggestions : uCLinux (but need 8MB for kernel only), why not ?