It seems like the default behaviour for spaces in custom bangs is to add a dash between the space terms. For instance, the term "!bang Two Words" seems to always be sent as "two-words".
While this is helpful for many search engines and sites, I'm trying to set one up that specifically needs %20 spacing - to be really specific to my case, using a bang to hop to a page on dota2protracker.com would be incredible, as it uses /hero/hero%20name to generate its pages on the backend - but this means that "Storm Spirit" for instance comes out as "storm-spirit", breaking the site. This isn't even really an intended use of bangs, as I'm not using it to search, but actually to jump around specific pages, but it's so close to perfection already, and this tiny change will make an overwhelming change to my life.