> Has SNFS been used for any numbers other than those of the form
> b^n+-1 and their factors? Could it be?
Yes to both. It can be used for anything which can be expressed as a
"small" polynomial. Look at
and links therein (especially the progress files) and you'll find a number
of examples of factoring (generalized) Cullen and Woodall numbers by SNFS.
To give just one example, Cullen(510) = 510*2^510+1 was factored with the
polynomial 510*m^5 + 1, where m=2^102.