This unfortunately can't be done unless mIRC creates a new quoted string construct. The idea of space tokenization is deeply rooted all the way down to the parser.. spaces are what mIRC uses to delimit commands and their respective arguments. $1- could never hold multiple spaces because it's tokenized by spaces to represent each word (or arguments, depending on context). Again, it's a deeply rooted principle of the language. $1- could only preserve spaces if something like the literal "hello world" (quotes included) counted as one token. Although that would be my suggestion, (I doubt it would break that many scripts), it's unlikely to be implemented.
You can always use $editbox($active) to get the non-space-modified data in the editbox if that's all you're looking for (since identifiers tokenize by commas, not spaces), but you could never use it in any commands, so it wouldn't help much.
