The advantage to starting with the Aeroplan site is that it is automatically calculated to determine routings that are within the Aeroplan 5% MPM restrictions.
Before coming up with crazy-all-over-the-place trips, if you verify the routing on the aeroplan site, chances are agents will have no problem booking it.
You can then search the individual flights for space through any booking database avaiable (Pseudo Nim, UA, AC, ANA, KVS etc)...
If you start with UA, which allows 15% MPM (I believe), there exists the possibility of space available, but not valid to be booked with an Aeroplan ticket.
Disclaimer: This can be a bit challenging for the mini RTW- check the other thread for mostly valid routings. (Some are allowed accidentally).