...My avatar voices my opinion on the whole "I-49 South" issue. 
Anyway. As for I-910 not being signed, I believe that Louisiana couldn't be bothered to sign it, since (in their view) they'll have to change the shields in a couple of years from 910 to 49. In their eyes, better to just wait til they can hang the 49 shields.
(What none of them in Baton Rouge seem to realize is that things take forever to get built around here, so honestly, I don't see I-49 getting signed in New Orleans in my lifetime. They may as well hang the damn 910 shields.)
First off, they can't post those I-910 shields on the Westbank Expressway until they get to finishing elevating the WB all the way to US 90...due to Fed rules about an Interstate spur ending on a US or NHS system highway.
Secondly...renaming proposed I-49 South as I-6 would still create a bit of a problem with the section through Lafayette, which is still mostly N-S and also carries a segment of US 167 (which I-49 overlays between Lafayette and just north of Opelousas). Personally, I'd rather that they keep the I-49 designation for continunity's sake...unless the Lafayette Metro Expressway outer toll loop on the southern and western sides of Lafayette is also built. Then you can route I-6 through there and the rest of US 90 just south and east of Lafayette, and keep I-49 for the urban portion through Lafayette.
And third...given the money issues, and the fact that the locals are now seriously talking about truncating I-49 South to overlay I-310 and end just west of New Orleans, they might just have to revive those I-910 shields...if they ever get to completing the Westbank Expressway, that is.
Anthony