Also, if to call it from SV or not... I'd say if basic connect() is ok from user mode, lwip is as well, remember Didier integrated this into TOS, it should work like any other call.
I couldn't resist, I was curios how it's done and actually, it's completely another way and now it makes even more sense. It's true Didier uses lwip in FireTOS but bridge to TOS isn't done by some wrapper in GEMDOS but via STiK compatible interface. So you must not use any lwip headers! You must use STiK/Sting API (via cookie).
Hope this helps.