I know this is an old topic but thought i'd reply as no-one else had. I'm fairly new to the whole programming/delphi thing so not sure if this solves/helps your problem:

a) What about using conditional defines for "debug" and "release" so when you build your package, it uses the desired extension. I'm not sure if the IDE will still silently replaces the line if it's inside the condition, i haven't tested it.

b) use a post-build batch file that just renames/copies the bpl to the filename you want. I don't know which IDE you're using, but i think it's in Project > Options somewhere.

Laters,
Ize