You develop on 3.9, then port and RELEASE on os4.0. In this way you can take part in the contest.
Then again, you forgot two simple things I stated before:
1) os3.9 and os4.0 APIs are quite similar. Os4.0 has some enhancements yes, but these doesn't broke retrocompatibility. It's somewhat like to port from linux 2.0 on i386 to linux 2.4 on i686.
2) many member of the grand jury of the contest have one (or more) A1 available and they're more than willing to help the contestants bugfixing their software before the deadline. So, the contestants do have that machine under the desk.
So, let me rephrase your statement: it is possible to work without easy access to the platform you're developing for. It's an Amiga, not your average PC.
Sorry, but it's not quite that simple. Believe me, I tried, it didn't work, so I bought an A1. Now, I haven't been working as hard on my project as I could have been but the porting of my old 3.9 code to 4.0 code is still in progress after 3 weeks, I've only skimmed the surface of the specific OS4 features.
Compiling a binary for OS4 is one thing, as Lando states, but then what... you give it to someone to test, it crashes, they tell you what and where, send you the Reaper Log, you send a new executable back which is possibly fixed, doesn't work... the circle continues. It is possible, but it's a pain in the backside!
Just because the APIs are 'similar' doesn't mean its just a matter of typing MAKE on OS4 with your 3.9 source code, it simply won't work. You could stuff a lot of #ifdef __amigaos_4__ in there I suppose.
Besides, if it's got to use ReAction how do you test that with a 3.9 app (ClassAct is not a solution here)? And use the OS4 specific features you're meant to be using as stated in the rules, application.library, AmiDock etc etc. Anyway, I'm a big fan of MUI and as it's in a native OS4 form now, I don't see why it should be banned.
I'm ranting... I'm off :-P