The new jump n instructions thingy is great, although it will be easy to break things by accident (one may not insert a line in the script without looking if you are breaking a jump. Also the counting vs. not counting of some instructions (SetCompress...) might become confusing. My suggestion is to introduce the new:
!LABEL <name> (or something in this direction).
Whenever a jump instruction occurs (which could still be specified in numbers if we want, you could optionally specify "ping" and the preprocessor would count the number of lines until !LABEL ping.
This would be done by the preprocessor and would beither increase the size of the installer .exe, but be less error prone.
What do you think?