A literary reference or a literal meaning is probably a bit too far-fetched, but in case no one else has any ideas and you have not considered these before, perhaps these could be suggestions that might lead us somewhere (but likely not).

The literal meaning of 降龍 is “to defeat dragons”. There is an idiom that you can find at http://dict.revised.moe.edu.tw/ (I can’t give an exact link, since that dictionary has a strange interface that makes it impossible to provide links to individual words) that alludes to surmounting problems. There is also a well-known novel with this term in its title.

If we follow any of these lines of thought, then 不降龍 would literally mean something to the effect of “there are dragons that can’t be defeated”; if this is in the right direction, it might mean something in the lines of “there are insurmountable problems”.

But as I’ve indicated in my confidence level, this is nothing but a wild guess.