Hi Benjamin,
You don't need "permission" from anybody, as long as you follow the provisions of the license. Nasm is under LGPL, the "lesser" or "library" GPL - as a .dll, the "library" license might actually make *sense*!
According to "internal.doc", Nasm was *intended* to be used in just such a manner. I seem to recall hearing that the "interface" (between modules, not the user interface) wasn't really quite right to do it, though... It will be interesting to see what, if any, problems you run into...
You might want to post your ideas to the "nasm-devel" list (the "lists" link, above), to see if anyone has any advice or suggestions for you. Someone *may* have done (or tried) this before - although I'm not aware of it.
But the license itself gives you all the "permission" you need. Go for it!!!
Best,
Frank